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Re: NO RAID5 <was: using raid 5 with a san >

Posted By:
Date: Thursday, 17 February 2005, at 7:21 p.m.

In Response To: NO RAID5 <was: using raid 5 with a san > (ART KAGEL, BLOOMBERG/ 731 LEXIN)

Thanks Art Kagel

ART KAGEL, .... wrote:

>OK, here's my quarterly NO RAID5 Rant, which explains why I oppose RAID5
>altogether and especially for Informix databases. If you need more ammo against
> the pointy of hair, see the BAARF web site, there are articles from well
>respected Oracle folk, in addition to my posting, there for you to see and use.
>Art S. Kagel
>RAID5 versus RAID10 (or even RAID3 or RAID4)
>What is RAID5?
>OK here is the deal, RAID5 uses ONLY ONE parity drive per stripe and many
>RAID5 arrays are 5 (if your counts are different adjust the calculations
>appropriately) drives (4 data and 1 parity though it is not a single
>drive that is holding all of the parity as in RAID 3 & 4 but read on). If
>you have 10 drives or say 20GB each for 200GB RAID5 will use 20% for
>parity so you will have 160GB of storage. Now since RAID10, like
>mirroring (RAID1), uses 1 (or more) mirror drive for each primary drive
>you areusing 50% for redundancy so to get the same 160GB of storage you
>will need 8 pairs or 16 - 20GB drives, which is why RAID5 is so popular.
>This intro is just to put things into perspective.
>RAID5 is physically a stripe set like RAID0 but with data recovery
>included. RAID5 reserves one disk block out of each stripe block for
>parity data. The parity block contains an error correction code which can
>correct any error in the RAID5 block, in effect it is used in combination
>with the remaining data blocks to recreate any single missing block, gone
>missing because a drive has failed. The innovation of RAID5 over RAID3 &
>RAID4 is that the parity is distributed on a round robin basis so that
>there can be independent reading of different blocks from the several
>drives. This is why RAID5 became more popular than RAID3 & RAID4 which
>must sychronously read the same block from all drives together. So, if
>Drive2 fails blocks 1,2,4,5,6 &7 are data blocks on this drive and blocks
>3 and 8 are parity blocks on this drive. So that means that the parity on
>Drive5 will be used to recreate the data block from Disk2 if block 1 is
>requested before a new drive replaces Drive2 or during the rebuilding of
>the new Drive2 replacement. Likewise the parity on Drive1 will be used to
>repair block 2 and the parity on Drive3 will repair block4, etc. For
>block 2 all the data is safely on the remaining drives but during the
>rebuilding of Drive2's replacement a new parity block will be calculated
>from the block 2 data and will be written to Drive 2.
>Now when a disk block is read from the array the RAID software/firmware
>calculates which RAID block contains the disk block, which drive the disk
>block is on and which drive contains the parity block for that RAID block
>and reads ONLY the data drive. It returns the data block . If you later
>modify the data block it recalculates the parity by subtracting the old
>block and adding in the new version then in two separate operations it
>writes the data block followed by the new parity block. To do this it
>must first read the parity block from whichever drive contains the parity
>for that stripe block and reread the unmodified data for the updated block
>from the original drive. This read-read-write-write is known as the RAID5
>write penalty since these two writes are sequential and synchronous the
>write system call cannot return until the reread and both writes complete,
>for safety, so writing to RAID5 is up to 50% slower than RAID0 for an
>array of the same capacity.
>Now what is RAID10:
>RAID10 is one of the combinations of RAID1 (mirroring) and RAID0
>(striping) which are possible. There used to be confusion about what
>RAID01 or RAID01 meant and different RAID vendors defined them differently.
>About five years or so ago I proposed the following standard language which
>seems to have taken hold. When N mirrored pairs are striped together
>this is called RAID10 because the mirroring (RAID1) is applied before
>striping (RAID0). The other option is to create two stripe sets and mirror
>them one to the other, this is known as RAID01 (because the RAID0 is applied
>first). In either a RAID01 or RAID10 system each and every disk block is
>completely duplicated on its drive's mirror. Performance-wise both RAID01
>and RAID10 are functionally equivalent. The difference comes in during
>recovery where RAID01 suffers from some of the same problems I will describe
>affecting RAID5 while RAID10 does not.
>Now if a drive in the RAID5 array dies, is removed, or is shut off data is
>returned by reading the blocks from the remaining drives and calculating
>the missing data using the parity, assuming the defunct drive is not the
>parity block drive for that RAID block. Note that it takes 4 physical
>reads to replace the missing disk block (for a 5 drive array) for four out
>of every five disk blocks leading to a 64% performance degradation until the
>problem is discovered and a new drive can be mapped in to begin recovery.
>If a drive in the RAID10 array dies data is returned from its mirror drive
>in a single read with only minor (6.25% on average) performance reduction
>when two non-contiguous blocks are needed from the damaged pair and none
>One begins to get an inkling of what is going on and why I dislike RAID5,
>but there's more.
>What's wrong besides a bit of performance I don't know I'm missing?
>OK, so that brings us to the final question of the day which is: What is
>the problem with RAID5? It does recover a failed drive right? So writes
>are slower, I don't do enough writing to worry about it and the cache
>helps a lot also, I've got LOTS of cache! The problem is that despite the
>improved reliability of modern drives and the improved error correction
>codes on most drives, and even despite the additional 8 bytes of error
>correction that EMC puts on every Clariion drive disk block (if you are
>lucky enough to use EMC systems), it is more than a little possible that
>a drive will become flaky and begin to return garbage. This is known as
>partial media failure. Now SCSI controllers reserve several hundred disk
>blocks to be remapped to replace fading sectors with unused ones, but if
>the drive is going these will not last very long and will run out and SCSI
>does NOT report correctable errors back to the OS! Therefore you will not
>know the drive is becoming unstable until it is too late and there are no
>more replacement sectors and the drive begins to return garbage. [Note
>that the recently popular ATA drives do not (TMK) include bad sector
>remapping in their hardware so garbage is returned that much sooner.]
>When a drive returns garbage, since RAID5 does not EVER check parity on
>read (RAID3 & RAID4 do BTW and both perform better for databases than
>RAID5 to boot) when you write the garbage sector back garbage parity will
>be calculated and your RAID5 integrity is lost! Similarly if a drive
>fails and one of the remaining drives is flaky the replacement will be
>rebuilt with garbage also.
>Need more? During recovery, read performance for a RAID5 array is degraded
>by as much as 80%. Some advanced arrays let you configure the preference
>more toward recovery or toward performance. However, doing so will
>increase recovery time and increase the likelihood of losing a second
>drive in the array before recovery completes resulting in catastrophic
>data loss. RAID10 on the other hand will only be recovering one drive out
>of 4 or more pairs with performance ONLY of reads from the recovering pair
>degraded making the performance hit to the array overall only about 20%!
>Plus there is no parity calculation time used during recovery - it's a
>straight data copy.
>What about that thing about losing a second drive? Well with RAID10 there
>is no danger unless the one mirror that is recovering also fails and
>that's 80% or more less likely than that any other drive in a RAID5 array
>will fail! And since most multiple drive failures are caused by
>undetected manufacturing defects you can make even this possibility
>vanishingly small by making sure to mirror every drive with one from a
>different manufacturer's lot number. ("Oh", you say, "this schenario does
>not seem likely!" Pooh, we lost 50 drives over two weeks when a batch of
>200 IBM drives began to fail. IBM discovered that the single lot of
>drives would have their spindle bearings freeze after so many hours of
>operation. Fortunately due in part to RAID10 and in part to a herculean
>effort by DG techs and our own people over 2 weeks no data was lost.
>HOWEVER, one RAID5 filesystem was a total loss after a second drive failed
>during recover. Fortunately everything was on tape.
>Conclusion? For safety and performance favor RAID10 first, RAID3 second,
>RAID4 third, and RAID5 last! The original reason for the RAID2-5 specs
>was that the high cost of disks was making RAID1, mirroring, impractical.
>That is no longer the case! Drives are commodity priced, even the biggest
>fastest drives are cheaper in absolute dollars than drives were then and
>cost per MB is a tiny fraction of what it was. Does RAID5 make ANY sense
>anymore? Obviously I think not.
>To put things into perspective: If a drive costs $1000US (and most are
>far less expensive than that today) then switching
>from a 4 pair RAID10 array to a 5 drive RAID5 array will save 3 drives or
>$3000US. What is the cost of overtime, wear and tear on the technicians,
>DBAs, managers, and customers of even a recovery scare? What is the cost
>of reduced performance and possibly reduced customer satisfaction? Finally
>what is the cost of lost business if data is unrecoverable? I maintain
>that the drives are FAR cheaper! Hence my mantra:
>Art S. Kagel


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