Quarterly RAID5 Rant - enhanced

Well, it's that time of year again. Here 'tis: 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 otherwise. 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 errors begin to exceed the drive's ECC coding's ability to correct single bit per byte errors, and the drive begins to return garbage. [Note that the recently popular ATA drives do not (to my knowledge) include bad sector remapping in their firmware 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

Interesting and comprehensive discussion. But I couldn't help noticing that in the Microsoft Guide To Get SQL Server Certification" (and let's resist any parochial pettiness in this debate) there were some sample questions on "Which RAID level would you choose in the following scenarios" and one of the answers was RAID 5. Although I shuddered at the answer myself, (and would probably have put something else and therefore lost a mark), and whatever we might say about Microsoft or SQL Server, it's interesting to note that an organisation with the wellie of Microsoft is still recommending RAID 5 to its up-and-coming DBAs. Also I was in the fortunate position to be able to try configuring IDS 9.3 on a big Sun with twin T3 arrays last year, tried RAID5 on one and RAID10 on the other and was surprised to find that both read and write performance were almost identical. Now you know I'm with you on your disdain for RAID5 Art but I'd be interested to hear your (or anyone else's) feedback on all of this. As I said it would be nice if we could keep the parochial stuff out of the debate. Andy