On Fri, Jan 29, 2010 at 3:53 AM, Goswin von Brederlow [off-list ref] wrote:

Michael Evans [off-list ref] writes:

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On Thu, Jan 28, 2010 at 7:27 AM, Robin Hill [off-list ref] wrote:

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On Thu Jan 28, 2010 at 09:55:05AM -0500, Yuehai Xu wrote:

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2010/1/28 Gabor Gombas [off-list ref]:

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On Thu, Jan 28, 2010 at 09:31:23AM -0500, Yuehai Xu wrote:

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md0 : active raid5 sdh1[7] sdg1[5] sdf1[4] sde1[3] sdd1[2] sdc1[1] sdb1[0]
      631353600 blocks level 5, 64k chunk, algorithm 2 [7/6] [UUUUUU_]

[...]

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I don't think any of my drive fail because there is no "F" in my
/proc/mdstat output

It's not failed, it's simply missing. Either it was unavailable when the
array was assembled, or you've explicitely created/assembled the array
with a missing drive.

I noticed that, thanks! Is it usual that at the beginning of each
setup, there is one missing drive?

Yes - in order to make the array available as quickly as possible, it is
initially created as a degraded array.  The recovery is then run to
add in the extra disk.  Otherwise all disks would need to be written
before the array became available.

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How do you know my RAID5 array has one drive missing?

Look at the above output: there are just 6 of the 7 drives available,
and the underscore also means a missing drive.

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I tried to setup RAID5 with 5 disks, 3 disks, after each setup,
recovery has always been done.

Of course.

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However, if I format my md0 with such command:
mkfs.ext3 -b 4096 -E stride=16 -E stripe-width=*** /dev/XXXX, the
performance for RAID5 becomes usual, at about 200~300M/s.

I suppose in that case you had all the disks present in the array.

Yes, I did my test after the recovery, in that case, does the "missing
drive" hurt the performance?

If you had a missing drive in the array when running the test, then this
would definitely affect the performance (as the array would need to do
parity calculations for most stripes).  However, as you've not actually
given the /proc/mdstat output for the array post-recovery then I don't
know whether or not this was the case.

Generally, I wouldn't expect the RAID5 array to be that much slower than
a RAID0.  You'd best check that the various parameters (chunk size,
stripe cache size, readahead, etc) are the same for both arrays, as
these can have a major impact on performance.

Cheers,
   Robin
--
    ___
   ( ' }     |       Robin Hill        [off-list ref] |
  / / )      | Little Jim says ....                            |
 // !!       |      "He fallen in de water !!"                 |

A more valid test that could be run would follow:

Assemble all the test drives as a raid-5 array (you can zero the
drives any way you like and then --assume-clean if they really are all
zeros) and let the resync complete.

Run any tests you like.

Stop and --zero-superblock on the array.

Create a striped array (raid 0) using all but one of the test drives.

Since you dropped the drive's worth of storage that would be dedicated
to parity in the raid-5 setup you're now benchmarking the same number
of /data/ storage drives; but have saved one drive's worth of recovery
data (at cost of risking your data if any single drive fails).

Still, run the same benchmarks.

Why is this valid instead of throwing all the drives at it in raid-0
mode as well?  It provides the same resulting storage size.


What I suspect you'll find is very similar read performance and
measurably, though perhaps tolerable, worse write performance from
raid-5.

In raid5 mode each drive will read 5*64k data and then skip 64k and
repeat. And skipping such a small chunk of data means waiting till it
has rotated below the head. So each drive only gives 5/6th of its linear
speed. As a result the 6 disks raid5 should be 5/6th of the speed of a 5
disk raid0 assuming the controler and bus are fast enough.

A larger chunk size can mean skipping the parity chunk skips a
cylinder. But larger chunk size makes it less likely reads are spread
over all/multiple disks. So you might loose more than you gain.

MfG
       Goswin

That is true assuming a very large sequential read (buffered video
streams and other very large files).  However while each drive will
only have an apparent performance of 5/6 in the case of a 6 drive raid
6 array that is still 5/6 * 6 = 5 drives raid zero equivalent; which
is also the size of usable data storage.  All the more reason to say:

Read performance of N+1 drives in raid 5 should be roughly equivalent
to N drives in raid 0 (obviously in the best case).

In the worst case, raid 5 produces data; while raid 0 times out and
fails to read any data.

The main area of performance difference between raid 5 and raid 0 is
seen on /writes/ which is where you pay for the insurance in the
complexity of keeping the stripe clean.  At /least/ reading any
changed chunks, plus parity chunk, calculating parity, and writing all
of that back to the drives; OR writing all the data-chunks and the
newly calculated parity chunk.  Thus describing why larger writes see
less overall degrade in performance and smaller writes seem so much
worse in comparison.  There's also the extra drive used as insurance.
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