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# from http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt
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# also see http://www.miracleas.com/BAARF/BAARF2.html
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# Note: I, the Debian maintainer, do not agree with some of the arguments,
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# especially not with the total condemning of RAID5. Anyone who talks about
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# data loss and blames the RAID system should spend time reading up on Backups
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# instead of trying to evangelise, but that's only my opinion. RAID5 has its
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# merits and its shortcomings, just like any other method. However, the author
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# of this argument puts forth a good case and thus I am including the
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# document. Remember that you're the only one that can decide which RAID level
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RAID5 versus RAID10 (or even RAID3 or RAID4)
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First let's get on the same page so we're all talking about apples.
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OK here is the deal, RAID5 uses ONLY ONE parity drive per stripe and many
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RAID5 arrays are 5 (if your counts are different adjust the calculations
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appropriately) drives (4 data and 1 parity though it is not a single drive
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that is holding all of the parity as in RAID 3 & 4 but read on). If you
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have 10 drives or say 20GB each for 200GB RAID5 will use 20% for parity
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(assuming you set it up as two 5 drive arrays) so you will have 160GB of
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storage. Now since RAID10, like mirroring (RAID1), uses 1 (or more) mirror
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drive for each primary drive you are using 50% for redundancy so to get the
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same 160GB of storage you will need 8 pairs or 16 - 20GB drives, which is
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why RAID5 is so popular. This intro is just to put things into
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RAID5 is physically a stripe set like RAID0 but with data recovery
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included. RAID5 reserves one disk block out of each stripe block for
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parity data. The parity block contains an error correction code which can
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correct any error in the RAID5 block, in effect it is used in combination
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with the remaining data blocks to recreate any single missing block, gone
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missing because a drive has failed. The innovation of RAID5 over RAID3 &
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RAID4 is that the parity is distributed on a round robin basis so that
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there can be independent reading of different blocks from the several
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drives. This is why RAID5 became more popular than RAID3 & RAID4 which
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must sychronously read the same block from all drives together. So, if
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Drive2 fails blocks 1,2,4,5,6 & 7 are data blocks on this drive and blocks
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3 and 8 are parity blocks on this drive. So that means that the parity on
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Drive5 will be used to recreate the data block from Disk2 if block 1 is
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requested before a new drive replaces Drive2 or during the rebuilding of
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the new Drive2 replacement. Likewise the parity on Drive1 will be used to
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repair block 2 and the parity on Drive3 will repair block4, etc. For block
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2 all the data is safely on the remaining drives but during the rebuilding
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of Drive2's replacement a new parity block will be calculated from the
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block 2 data and will be written to Drive 2.
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Now when a disk block is read from the array the RAID software/firmware
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calculates which RAID block contains the disk block, which drive the disk
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block is on and which drive contains the parity block for that RAID block
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and reads ONLY the one data drive. It returns the data block. If you
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later modify the data block it recalculates the parity by subtracting the
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old block and adding in the new version then in two separate operations it
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writes the data block followed by the new parity block. To do this it must
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first read the parity block from whichever drive contains the parity for
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that stripe block and reread the unmodified data for the updated block from
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the original drive. This read-read-write-write is known as the RAID5 write
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penalty since these two writes are sequential and synchronous the write
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system call cannot return until the reread and both writes complete, for
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safety, so writing to RAID5 is up to 50% slower than RAID0 for an array of
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the same capacity. (Some software RAID5's avoid the re-read by keeping an
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unmodified copy of the orginal block in memory.)
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RAID10 is one of the combinations of RAID1 (mirroring) and RAID0
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(striping) which are possible. There used to be confusion about what
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RAID01 or RAID01 meant and different RAID vendors defined them
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differently. About five years or so ago I proposed the following standard
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language which seems to have taken hold. When N mirrored pairs are
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striped together this is called RAID10 because the mirroring (RAID1) is
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applied before striping (RAID0). The other option is to create two stripe
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sets and mirror them one to the other, this is known as RAID01 (because
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the RAID0 is applied first). In either a RAID01 or RAID10 system each and
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every disk block is completely duplicated on its drive's mirror.
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Performance-wise both RAID01 and RAID10 are functionally equivalent. The
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difference comes in during recovery where RAID01 suffers from some of the
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same problems I will describe affecting RAID5 while RAID10 does not.
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Now if a drive in the RAID5 array dies, is removed, or is shut off data is
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returned by reading the blocks from the remaining drives and calculating
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the missing data using the parity, assuming the defunct drive is not the
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parity block drive for that RAID block. Note that it takes 4 physical
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reads to replace the missing disk block (for a 5 drive array) for four out
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of every five disk blocks leading to a 64% performance degradation until
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the problem is discovered and a new drive can be mapped in to begin
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recovery. Performance is degraded further during recovery because all
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drives are being actively accessed in order to rebuild the replacement
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If a drive in the RAID10 array dies data is returned from its mirror drive
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in a single read with only minor (6.25% on average for a 4 pair array as a
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whole) performance reduction when two non-contiguous blocks are needed from
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the damaged pair (since the two blocks cannot be read in parallel from both
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drives) and none otherwise.
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One begins to get an inkling of what is going on and why I dislike RAID5,
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but, as they say on late night info-mercials, there's more.
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What's wrong besides a bit of performance I don't know I'm missing?
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OK, so that brings us to the final question of the day which is: What is
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the problem with RAID5? It does recover a failed drive right? So writes
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are slower, I don't do enough writing to worry about it and the cache
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helps a lot also, I've got LOTS of cache! The problem is that despite the
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improved reliability of modern drives and the improved error correction
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codes on most drives, and even despite the additional 8 bytes of error
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correction that EMC puts on every Clariion drive disk block (if you are
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lucky enough to use EMC systems), it is more than a little possible that a
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drive will become flaky and begin to return garbage. This is known as
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partial media failure. Now SCSI controllers reserve several hundred disk
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blocks to be remapped to replace fading sectors with unused ones, but if
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the drive is going these will not last very long and will run out and SCSI
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does NOT report correctable errors back to the OS! Therefore you will not
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know the drive is becoming unstable until it is too late and there are no
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more replacement sectors and the drive begins to return garbage. [Note
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that the recently popular IDE/ATA drives do not (TMK) include bad sector
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remapping in their hardware so garbage is returned that much sooner.]
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When a drive returns garbage, since RAID5 does not EVER check parity on
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read (RAID3 & RAID4 do BTW and both perform better for databases than
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RAID5 to boot) when you write the garbage sector back garbage parity will
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be calculated and your RAID5 integrity is lost! Similarly if a drive
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fails and one of the remaining drives is flaky the replacement will be
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rebuilt with garbage also propagating the problem to two blocks instead of
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Need more? During recovery, read performance for a RAID5 array is
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degraded by as much as 80%. Some advanced arrays let you configure the
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preference more toward recovery or toward performance. However, doing so
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will increase recovery time and increase the likelihood of losing a second
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drive in the array before recovery completes resulting in catastrophic
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data loss. RAID10 on the other hand will only be recovering one drive out
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of 4 or more pairs with performance ONLY of reads from the recovering pair
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degraded making the performance hit to the array overall only about 20%!
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Plus there is no parity calculation time used during recovery - it's a
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What about that thing about losing a second drive? Well with RAID10 there
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is no danger unless the one mirror that is recovering also fails and
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that's 80% or more less likely than that any other drive in a RAID5 array
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will fail! And since most multiple drive failures are caused by
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undetected manufacturing defects you can make even this possibility
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vanishingly small by making sure to mirror every drive with one from a
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different manufacturer's lot number. ("Oh", you say, "this schenario does
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not seem likely!" Pooh, we lost 50 drives over two weeks when a batch of
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200 IBM drives began to fail. IBM discovered that the single lot of
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drives would have their spindle bearings freeze after so many hours of
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operation. Fortunately due in part to RAID10 and in part to a herculean
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effort by DG techs and our own people over 2 weeks no data was lost.
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HOWEVER, one RAID5 filesystem was a total loss after a second drive failed
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during recover. Fortunately everything was on tape.
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Conclusion? For safety and performance favor RAID10 first, RAID3 second,
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RAID4 third, and RAID5 last! The original reason for the RAID2-5 specs
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was that the high cost of disks was making RAID1, mirroring, impractical.
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That is no longer the case! Drives are commodity priced, even the biggest
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fastest drives are cheaper in absolute dollars than drives were then and
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cost per MB is a tiny fraction of what it was. Does RAID5 make ANY sense
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anymore? Obviously I think not.
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To put things into perspective: If a drive costs $1000US (and most are far
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less expensive than that) then switching from a 4 pair RAID10 array to a 5
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drive RAID5 array will save 3 drives or $3000US. What is the cost of
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overtime, wear and tear on the technicians, DBAs, managers, and customers
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of even a recovery scare? What is the cost of reduced performance and
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possibly reduced customer satisfaction? Finally what is the cost of lost
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business if data is unrecoverable? I maintain that the drives are FAR
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cheaper! Hence my mantra:
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NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5!
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docs/RAID5_versus_RAID10.txt
