On 1/1/2014 12:50 AM, Tomas M wrote:

My main concern is that I'm not always 100% sure if a certain drive is
the root cause of slow read performance, so failing a wrong one would
have terrible consequences. On the other hand, if it was possible to
say "read the data with pretending one drive has failed, but write to
the array as if all drives are ok", then I could actually find out if
copying speed reverts to normal.

So you want to monkey with the way RAID works just to figure out if one
of your drives is flaky, causing slow IO?

Tools already exist to identify such things.  "iostat -x" 'await' shows
latency per drive.  "smartctl -A /dev/device" gives you
Raw_Read_Error_Rate and Seek_Error_Rate.   High counts in the RAW_VALUE
column for either indicates the drive is re-reading sectors and applying
extra DSP to figure out the magnetism of the bits.

Comparing such data amongst your array member drives should pretty
quickly tell you if one has a problem.  If none do, you need to look
elsewhere for the problem.

Your initial post suggested you knew which drive was flaky.  Now you
indicate you don't know which, if any, is flaky.  This suggests you have
no idea why your array is slow.

The first place to look when IO slows down suddenly is the filesystem.
Nearly full filesystems scatter extents into any free space available
when writing new files or appending files.  When you read these back you
get far more seeking than normal and everything slows down,
dramatically.  Investigate this possibility, and the things above,
before attempting to rewrite raid5.c, which will inevitably cause you
more problems than it solves.

-- 
Stan

Maybe I should write a kernel patch to try this out. Any suggestions
where to start?

Tomas M

On Tue, Dec 31, 2013 at 4:17 PM, Stan Hoeppner [off-list ref] wrote:

quoted

On 12/31/2013 8:42 AM, Tomas M wrote:

quoted

I'm using software raid 5 (stripe with one parity drive).

Is there a way to force the raid array to MOSTLY ignore one of the drives?

Let me explain. If a drive is failing (is very slow, has errors, etc),
then I still prefer to keep it in the array and simply COPY all data
from the array somewhere else, instead of risking that the array gets
degraded if I remove the failing drive and another one dies at the
same time.

Example:
- 4 drives in raid5
- one drive is slow, lets call the drive DISK1
- copying all data from the array is very slow because it still uses
DISK1 to read data from it, even if it could IGNORE it and COMPUTE the
data from the other three drives

The filesystem on the array needs to be still mounted rw, though,
since there may be some changes.

Is there any mdadm parameter or option which would make the array
IGNORE any given disk on reads (since those can be computed from other
drives), while still NOT IGNORING the disk in writes?

Because if I set it to ignore that failing slow drive, I will copy out
the data 100 times faster while still keeping the array in sync.
Because for me if ANOTHER drive dies, it will still be better to have
the data 100 times slower than nothing. I hope you understand me :)

The option you request is not available TTBOMK.  So...

Fail the drive.  Copy all the data off the array.  Zero the drive and
add it back.  Resync.

But while you're at it, why not simply replace the flaky drive before
the resync?  Why would you intentionally keep a known-to-be-failing
drive in the array?

--
Stan

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