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This type of RAID is for companies like IBM or NCR.I think that whoever wrote RAID was surrounded by Military and they were patting him on the back.
Oh wait, wrong type of RAID...LOL 
Dual parity has been around for quite some time (sometimes referred to as data guarding by vendors like HP). I personally don't think it's needeed on servers per se, but it should be implemented on SAN arrays or NAS devices that host critical data.
Dual parity has been around for quite some time (sometimes referred to as data guarding by vendors like HP). I personally don't think it's needeed on servers per se, but it should be implemented on SAN arrays or NAS devices that host critical data.
At one previous employer, the cost of failure could potentially reach $15,750 per HOUR in an extreme case, and this wasn't that large a business.
Any executive who sweats the cost of failsafe hardware and its trivial electrical demand is to my mind, an idiot. Our IT manager had older equipment as completely redundant servers, which could be brought online in short order. Was she paranoid, or wasteful ?
Not to my mind. Very practical considering the cost of failure. I'd recommend her approach any day.
Any executive who sweats the cost of failsafe hardware and its trivial electrical demand is to my mind, an idiot. Our IT manager had older equipment as completely redundant servers, which could be brought online in short order. Was she paranoid, or wasteful ?
Not to my mind. Very practical considering the cost of failure. I'd recommend her approach any day.
Your observation that going from RAID 5 to RAID 6 cuts your available capacity fro 80% down to 60% is a valid concern.
However, you then seem to recommend using mirroring - which guarantees that you will only get 50% capacity.
Even granting that rebuilding a mirror doesn't have nearly the performance hit of rebuilding a parity stripe your argument falls a little flat, especially when the later comments about OPEX are also added in.
However, you then seem to recommend using mirroring - which guarantees that you will only get 50% capacity.
Even granting that rebuilding a mirror doesn't have nearly the performance hit of rebuilding a parity stripe your argument falls a little flat, especially when the later comments about OPEX are also added in.
I agree here. This piece needs some reworking and reposting.
As stated, it's just plain wrong (20% capacity loss for RAID 5 - only with 5 disks! Less with more disks, duh), misleading (RAID 1+0 is worse on a lot of the factors the author brought up), and willfully obscure (CAPEX, OPEX? This isn't economics where we strive to use obscure terms for concepts everybody understands... this is a tech article so use the words, not acronyms for CIO, CFOs.)
I especially hate authors who state a fixed % in overhead on RAID 5. It just shows they don't have a good understanding about RAID 5 to begin with, or are lazy in their writing(state your assumptions). Either way, do I really want to trust that person?
Losing 1 disk on a 14 drive array is a negligible space hit, and only 7% loss!
As stated, it's just plain wrong (20% capacity loss for RAID 5 - only with 5 disks! Less with more disks, duh), misleading (RAID 1+0 is worse on a lot of the factors the author brought up), and willfully obscure (CAPEX, OPEX? This isn't economics where we strive to use obscure terms for concepts everybody understands... this is a tech article so use the words, not acronyms for CIO, CFOs.)
I especially hate authors who state a fixed % in overhead on RAID 5. It just shows they don't have a good understanding about RAID 5 to begin with, or are lazy in their writing(state your assumptions). Either way, do I really want to trust that person?
Losing 1 disk on a 14 drive array is a negligible space hit, and only 7% loss!
Yeah, the author assumes a 5-disk RAID array, but it's easy enough to ignore minor issues to look at the overall article content. Nitpicking is a worse habit than simply ignoring the minor details. You're ready to throw the whole article out (and the article does a good job giving an overview of RAID6) on a minor technicality and...
...your own ignorance. If you're working in IT and you don't know what capex and opex are, you really need to consider another field. IT isn't about technology, it's about meeting the needs of the business. If you don't understand that and those needs, you and your business are destined to fail. As an IT person you are REQUIRED to have an understanding of economics, business management, payroll, HR, accounting, manufacturing, etc, etc, etc. It's not about you getting the newest toys to play with on someone else's dime.
...your own ignorance. If you're working in IT and you don't know what capex and opex are, you really need to consider another field. IT isn't about technology, it's about meeting the needs of the business. If you don't understand that and those needs, you and your business are destined to fail. As an IT person you are REQUIRED to have an understanding of economics, business management, payroll, HR, accounting, manufacturing, etc, etc, etc. It's not about you getting the newest toys to play with on someone else's dime.
I'm sure that you know all about those "needs"! I don't know what are "capex" and "opex" also, but if I will want to know I will find some details about these.
Since the minimum for RAID 5 is 3 disks, does RAID 6 require 4 disks?
My math skills calculate you are using 5 disks in your RAID array since you have 80% usability with RAID 5 and 60% with RAID 6. I think that would be 75% in RAID 5 and 50% using 4 disks.
Hot spares sound better. Or maybe you need to be buying more dependable disks.
My math skills calculate you are using 5 disks in your RAID array since you have 80% usability with RAID 5 and 60% with RAID 6. I think that would be 75% in RAID 5 and 50% using 4 disks.
Hot spares sound better. Or maybe you need to be buying more dependable disks.
It is that very paragraph that this author's article spiralled
out of control into nothing more than worthless fear-
mongering and shockformation.
Facts: RAID5 incurs a single disks worth of added physical
write penalty. RAID6 incurs two disks worth. For this
reason, the physical write speed penalty for a RAID5 array
is 50% of the speed of a single disk; we must
simultaneously write to the active block disk, and to the
parity block disk. The RAID6 array incurs a 66% write
penalty, writing simultaneously to the active block disk,
the first-level parity block disk, and the second-level
parity block disk. This write speed penalty is mitigated by
write-back caching on most controllers in a UPS-
assured system but the maximum physical write speed
cannot be overcome. If write performance required
cannot sustain this penalty, and if high-availability is an
absolute requirement, then multi-level RAID is the only
option.
Next, the arbitrary % disk loss figures quoted are absurd.
You are correct that the author is using 5 disks, which is
unheard of in the enterprise. The most common RAID5
implementation being purchased in the mid-level sector is
the Apple XRaid shelf at 6 data drives and 1 drive for hot-
spare (XRaid does not support RAID6). This physical
layout incurs -as all RAID5 implentations using a
hotspare- an N-2 space penalty and comes to 5/7, or
71.4%.
RAID6, if even possible on the XRaid, would typically incur
the exact same capacity penalty as RAID5 since a hot
spare is not normally used (since the need to immediately
begin rebuilding the array is not as critical since a second
disk failure during rebuild (which is not uncommon) will
not incur data loss. If however, we did want the
exponentially-greater data protection offered by a hot-
spare in a RAID6 implementation of 7 total disks, the
capacity penalty would be N-3, or 4/7, or 57.1%
Now let's look at the most common RAID6
implementations addressing simultaneously the power,
datacenter space, raw capacity, and data protection...
these are typically 12 to 16 member stripes with 83.3% to
87.5% capacity efficiency respectively. RAID5 would incur
the same rates, but lets say we used no hotspare (gasp),
the rates would be 91.6% to 93.8% with less data
protection in a failure condition than RAID0. No thank
you.
Nexsan offers a product that currently can provide a 42-
member RAID6 stripe SATA-based array with a raw
capacity of N-2 where N=750GB (that's 30TB flat my
friend). This is with a heat signature of 570W (single
controller model) and 4U of rack space. That equates to
19W/TB (19mW/GB) and 7.5TB/U (7500GB/U). Raw
numbers for this configuration show block-level write
speeds of 128MB/s. The theoretical maximum speed of
2Gbps Fibrechannel is 250MB/s so the worst streaming
write-speed penalty is approximately 50% over RAID10
(assuming RAID10 could saturate the Fibrechannel in this
scenario, which is likely).
Bottom line, is there is no Swiss Army Knife in storage.
Everything is a comprimise. Protection, Performance,
Cost... pick 2. As performance of drives today is at least
double those of just 5 years ago (extremely conservative
estimate) the penalty for RAID6 vs RAID5 is increasingly
becoming an acceptible penalty for protection of very
critical data, or (more likely) the SMB market that does not
have the luxury of being able to sustain the complete loss
of such large amounts of data for the time it would
require to restore completely from backup. Business
continuity is paramount to performance in the majority of
this market.
My personal opinion is that if your performace margin is
so thin that you are forced to choose a RAID5
implementation over a RAID6, then you are not designing
a sustainable or scalable system with the best tools
available today and should consider a different physical
storage model (such as RAID10 or RAID60) over putting
data protection and business continuity at risk.
One last comment regarding "you need to be buying more
dependable disks", there are two kinds of people in the
world: those that have had a hard drive crash... and those
that will.
-=dave
[edit: fixed some typos]
out of control into nothing more than worthless fear-
mongering and shockformation.
Facts: RAID5 incurs a single disks worth of added physical
write penalty. RAID6 incurs two disks worth. For this
reason, the physical write speed penalty for a RAID5 array
is 50% of the speed of a single disk; we must
simultaneously write to the active block disk, and to the
parity block disk. The RAID6 array incurs a 66% write
penalty, writing simultaneously to the active block disk,
the first-level parity block disk, and the second-level
parity block disk. This write speed penalty is mitigated by
write-back caching on most controllers in a UPS-
assured system but the maximum physical write speed
cannot be overcome. If write performance required
cannot sustain this penalty, and if high-availability is an
absolute requirement, then multi-level RAID is the only
option.
Next, the arbitrary % disk loss figures quoted are absurd.
You are correct that the author is using 5 disks, which is
unheard of in the enterprise. The most common RAID5
implementation being purchased in the mid-level sector is
the Apple XRaid shelf at 6 data drives and 1 drive for hot-
spare (XRaid does not support RAID6). This physical
layout incurs -as all RAID5 implentations using a
hotspare- an N-2 space penalty and comes to 5/7, or
71.4%.
RAID6, if even possible on the XRaid, would typically incur
the exact same capacity penalty as RAID5 since a hot
spare is not normally used (since the need to immediately
begin rebuilding the array is not as critical since a second
disk failure during rebuild (which is not uncommon) will
not incur data loss. If however, we did want the
exponentially-greater data protection offered by a hot-
spare in a RAID6 implementation of 7 total disks, the
capacity penalty would be N-3, or 4/7, or 57.1%
Now let's look at the most common RAID6
implementations addressing simultaneously the power,
datacenter space, raw capacity, and data protection...
these are typically 12 to 16 member stripes with 83.3% to
87.5% capacity efficiency respectively. RAID5 would incur
the same rates, but lets say we used no hotspare (gasp),
the rates would be 91.6% to 93.8% with less data
protection in a failure condition than RAID0. No thank
you.
Nexsan offers a product that currently can provide a 42-
member RAID6 stripe SATA-based array with a raw
capacity of N-2 where N=750GB (that's 30TB flat my
friend). This is with a heat signature of 570W (single
controller model) and 4U of rack space. That equates to
19W/TB (19mW/GB) and 7.5TB/U (7500GB/U). Raw
numbers for this configuration show block-level write
speeds of 128MB/s. The theoretical maximum speed of
2Gbps Fibrechannel is 250MB/s so the worst streaming
write-speed penalty is approximately 50% over RAID10
(assuming RAID10 could saturate the Fibrechannel in this
scenario, which is likely).
Bottom line, is there is no Swiss Army Knife in storage.
Everything is a comprimise. Protection, Performance,
Cost... pick 2. As performance of drives today is at least
double those of just 5 years ago (extremely conservative
estimate) the penalty for RAID6 vs RAID5 is increasingly
becoming an acceptible penalty for protection of very
critical data, or (more likely) the SMB market that does not
have the luxury of being able to sustain the complete loss
of such large amounts of data for the time it would
require to restore completely from backup. Business
continuity is paramount to performance in the majority of
this market.
My personal opinion is that if your performace margin is
so thin that you are forced to choose a RAID5
implementation over a RAID6, then you are not designing
a sustainable or scalable system with the best tools
available today and should consider a different physical
storage model (such as RAID10 or RAID60) over putting
data protection and business continuity at risk.
One last comment regarding "you need to be buying more
dependable disks", there are two kinds of people in the
world: those that have had a hard drive crash... and those
that will.
-=dave
[edit: fixed some typos]
Dave,
Your 1st paragraph lost me. You seem to base your % on a 2 disk RAID 5. Wouldn't that be very unusual?
Your 1st paragraph lost me. You seem to base your % on a 2 disk RAID 5. Wouldn't that be very unusual?
The % in the first paragraph is the write penalty at the
block level. While the array may consist of many disks
(the stripe) the writes occur at the block level and by
definition a block cannot span disks. For a single stripe
in an array using a RAID5 implementation of 11 disks,
there would exist 10 blocks of data on 10 disks, and 1
block of parity on 1 disk. For a single write consisting of
the 1-byte char "A" to the array, 1 block on 1 of the 10
data block disks will be written to, and the 1 disk with the
parity block will be written to. The same write to a RAID6
array of 12 disks would write to 1 of the 10 data block
disks, and 2 separate blocks to the 2 parity blocks.
I hope this makes more sense.
-=dave
block level. While the array may consist of many disks
(the stripe) the writes occur at the block level and by
definition a block cannot span disks. For a single stripe
in an array using a RAID5 implementation of 11 disks,
there would exist 10 blocks of data on 10 disks, and 1
block of parity on 1 disk. For a single write consisting of
the 1-byte char "A" to the array, 1 block on 1 of the 10
data block disks will be written to, and the 1 disk with the
parity block will be written to. The same write to a RAID6
array of 12 disks would write to 1 of the 10 data block
disks, and 2 separate blocks to the 2 parity blocks.
I hope this makes more sense.
-=dave
RAID10 is better!
What is wrong with RAID10? - far better than RAID5 in every way - as disks are cheap the primary argument for using RAID5 (or 6) died in the late 90's!
What is wrong with RAID10? - far better than RAID5 in every way - as disks are cheap the primary argument for using RAID5 (or 6) died in the late 90's!
RAID 5E works like the descriptions in this article and I use it from years.
It needs a minimum of 4 hard disks and survive with two defect disks.
It needs a minimum of 4 hard disks and survive with two defect disks.
I think with RAID 5 and Online Spare, you can have peace of mind if the RAID fails and also with this setup there is no need for RAID 6.
What do you do if another disk in the array fails before the rebuild onto the on-line spare is complete? Start scrabbling for backups, that's what!
The justification for RAID 6 is something that is becoming much significant as the capacity of disks goes on doubling whilst the access speed doesn't change. I leave you to think on how long it's going to take to rebuild a 1000GB hot standby in a RAID 5. You are TOTALLY vulnerable for that time.
The justification for RAID 6 is something that is becoming much significant as the capacity of disks goes on doubling whilst the access speed doesn't change. I leave you to think on how long it's going to take to rebuild a 1000GB hot standby in a RAID 5. You are TOTALLY vulnerable for that time.
While an online spare allows the rebuild to start right away, it does not allow for a second failure during the rebuild process. It also does not benefit any in space as it will result in same space available as raid6. With multi-terabyte drives arrays, the rebuild time is in the almost a day to rebuild time frame. The chance on losing a second drive with a multi-drive raid set is not only possible but can be probable when using 14 drives in a raid set.
Having had experience with Multiple RAID environments from professional workstations running linux to terabyte arrays on Windows 2003 Servers and clustered across state lines (on fiber backbones ... and to you Brits it IS spelled fiber not fibre), such experience talks volumes to what actually works and what does not work in REAL LIFE practice.
Raid 5 with a Hot spare IS THE BEST OPTION, PROVIDED you use multi-stage imaged solutions, ISO imaging with RIB/ILO setups virtualizing critical servers on the latest clustered virtual solutions. Terabyte arrays will only work effectively on dynamic RAID configurations. DHCP/DNS/WINS systems can be booted with the latest Windows OS without the GUI to utilize all resources for the processes on virtualized domains as well as making sure the environment is completely flexible.
All these factors enable the admins to make use of such "good enough" RAID configurations due to the flexibility of the other solutions they have implemented.
Only use the RAID 5 with hot spare configurations on systems that do not have ANY data backup needs, like on servers that are members of DNS round robins, while utilizing HOME drive storage solutions on dynamic terabyte arrays.
The holistic (whole environment) solution approach to your data integrity assures known points of failure have dynamic contingencies.
Raid 5 with a Hot spare IS THE BEST OPTION, PROVIDED you use multi-stage imaged solutions, ISO imaging with RIB/ILO setups virtualizing critical servers on the latest clustered virtual solutions. Terabyte arrays will only work effectively on dynamic RAID configurations. DHCP/DNS/WINS systems can be booted with the latest Windows OS without the GUI to utilize all resources for the processes on virtualized domains as well as making sure the environment is completely flexible.
All these factors enable the admins to make use of such "good enough" RAID configurations due to the flexibility of the other solutions they have implemented.
Only use the RAID 5 with hot spare configurations on systems that do not have ANY data backup needs, like on servers that are members of DNS round robins, while utilizing HOME drive storage solutions on dynamic terabyte arrays.
The holistic (whole environment) solution approach to your data integrity assures known points of failure have dynamic contingencies.
At the very least, I want the option then I'll make the decisions as to the RAID level that I used based on the importance of the data.
My current SAN has a mix of fibre channel and SATA and for 300GB fibre channel disks, RAID 5 is just fine. For arrays made up of 800GB SATA, the rebuild time onto the hot spare can be over a day if the SAN is being hit hard as rebuild is very processor intensive and is of a lower priority than data reads and writes. For some crucial data which is on the cheaper SATA, because it's big data and also not hit too hard, I want - and have - the safety net of RAID 6. I use RAID 5 on SATA for stuff that isn't so important. 800GB fibre-attached SATA disks aren't so cheap that I can justify getting the 5TB that I need from a 10-disk raid 1+0 set when I can use a 7-disk RAID 6 set instead.
My current SAN has a mix of fibre channel and SATA and for 300GB fibre channel disks, RAID 5 is just fine. For arrays made up of 800GB SATA, the rebuild time onto the hot spare can be over a day if the SAN is being hit hard as rebuild is very processor intensive and is of a lower priority than data reads and writes. For some crucial data which is on the cheaper SATA, because it's big data and also not hit too hard, I want - and have - the safety net of RAID 6. I use RAID 5 on SATA for stuff that isn't so important. 800GB fibre-attached SATA disks aren't so cheap that I can justify getting the 5TB that I need from a 10-disk raid 1+0 set when I can use a 7-disk RAID 6 set instead.
Best alternative to this problem is RAID 0+1 But no all raid controllers support it
Actually, you could sustain irretreivable data lose the
moment you loose one member of a RAID10 array if during
rebuild data from the remaining member of the original
mirrored pair is unreadable. Only a dual-parity protection
mechanism can sustain a single disk failure with protection
from bad blocks during rebuild.
moment you loose one member of a RAID10 array if during
rebuild data from the remaining member of the original
mirrored pair is unreadable. Only a dual-parity protection
mechanism can sustain a single disk failure with protection
from bad blocks during rebuild.
Having a hot spare in any monolithic server or array is such a low cost, usually easy to implement in any configuration. Well worth the cost of an extra disk for any config...helps one sleep at night. I'd put in a hot spare even with a RAID 6.
I just had an argument with a software vendor where he demanded a raid-5 over a raid-1. I pointed out that in either case if one drive fails-still running but, also in either case if more than one drive failed - dead server. I was thinking that a raid-1 would outperform a raid-5. I ended up with a raid-1 boot and a raid-5 for data with a hot spare. I think that raid-5 WAS the original way to get more storage when drives were so expensive. For a 25-computer doctors office, they are currently using 500gb (6-years of patient data) so, I thought 1TB velociraptors would be the ticket - mirrored. right or wrong??
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