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Wednesday, June 16, 2010

RAID5 Rant

I used to post to the Informix NewsGroup about once a year.  I haven't done so in a long time, and now I have a different forum to do that in.  This BLOG.  So, for all of your reading pleasure here is my analysis of why RAID5 is Unsafe at Any Speed:


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.
Several years 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.

OK, so what?

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, as they say on late night info-mercials, wait, 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!", I can hear you say, "This schenario does not seem likely!" 
Unfortunately it is all too likely.  It happened to me and I have heard
from several other DBAs and SAs who have similar experiences.  My former
employer lost 50 drives over two weeks when a batch of 200 IBM OEM drives
began to fail.  IBM discovered that that 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 and the restore succeeded.  However, that filesystem
was down for several hours causing 1500 developers to twiddle their thumbs
for most of a day.  That one internal service outage of only a few hours
cost more in lost productivity than the extra cost of using RAID10 for all
of those filesystem arrays! 

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!  NO RAID5!  NO RAID5!  NO RAID5!