On 21/04/2009 02:33, NeilBrown wrote:
[...]
Choosing a larger --bitmap-chunk size will require fewer updates to the
bitmap before writes are allowed to proceed. However a larger bitmap-chunk
size will also increase the amount of work needed after a crash or
re-added device.
Ah, from my reading of the mdadm man page I thought you could only
specify the chunk size when using an external bitmap:
--bitmap-chunk=
Set the chunksize of the bitmap. Each bit corresponds to that
many Kilobytes of storage. When using a file based bitmap, the
default is to use the smallest size that is at-least 4 and
requires no more than 2^21 chunks. When using an internal
bitmap, the chunksize is automatically determined to make best
use of available space.
Check your current (default) chunk size with "mdadm -X /dev/sdxx" and
create a new bitmap with (say) 16 or 64 times the chunk size.
See if that makes a difference.
It certainly does. Upping it from 2M to 32M gets me from 45MB/s to
81MB/s on the scratch LV, and there's noticeably less seek noise.
Eeek! Trying to `mdadm --grow /dev/md1 --bitmap=none` from my large
chunk size caused a reboot! There's nothing in the log, and I didn't see
the console. I still have my 32M chunksize but I don't want to try that
again in a hurry :-)
[...]
Your other option is to put the bitmap in a file on some other device.
If you have a device this is rarely used (maybe your root filesystem)
Can't do that, my root filesystem is on the RAID-5, and part of the
reason for wanting the bitmap is because the md can't be stopped while
shutting down, so it was always wanting to resync at startup, which is
rather tedious.
Maybe you could create an external bitmap in a tmpfs.... but then
it wouldn't survive a crash and so has little value. It would be
fast though :-)
"Ooh, virtual memory! Now I can have a really big RAM disc!"
Now, it's time I checked my discs to see if I've lost data in that
crash. `mdadm -X` is stuck saying there are 10 dirty chunks.
Cheers,
John.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
