Posts tagged storage
In a earlier post I mentioned a linux readme where someone ranted on raid levels. As it seems, this one is no longer present, so here's a reprint.
# from http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt # also see http://www.miracleas.com/BAARF/BAARF2.html # # Note: I, the Debian maintainer, do not agree with some of the arguments, # especially not with the total condemning of RAID5. Anyone who talks about # data loss and blames the RAID system should spend time reading up on Backups # instead of trying to evangelise, but that's only my opinion. RAID5 has its # merits and its shortcomings, just like any other method. However, the author # of this argument puts forth a good case and thus I am including the # document. Remember that you're the only one that can decide which RAID level # to use. #
RAID5 versus RAID10 (or even RAID3 or RAID4)
First let's get on the same page so we're all talking about apples.
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 (assuming you set it up as two 5 drive arrays) so you will have 160GB of storage. Now since RAID10, like mirroring (RAID1), uses 1 (or more) mirror drive for each primary drive you are using 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 one 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. (Some software RAID5's avoid the re-read by keeping an unmodified copy of the orginal block in memory.)
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 RAID10 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. Performance is degraded further during recovery because all drives are being actively accessed in order to rebuild the replacement drive (see below).
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 for a 4 pair array as a whole) performance reduction when two non-contiguous blocks are needed from the damaged pair (since the two blocks cannot be read in parallel from both drives) and none otherwise.
One begins to get an inkling of what is going on and why I dislike RAID5, but, as they say on late night info-mercials, 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 IDE/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 propagating the problem to two blocks instead of just one.
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) 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:
NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5!
Art S. Kagel
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