On 12/09/2012 13:52, Adam Goryachev wrote:
On 11/09/12 00:12, Ralf Müller wrote:
Besides all the stuff about fix your server, a raid is not a backup and you risk your data - simply add a write intent bitmap:
mdadm /dev/md2 --grow --bitmap=internal
I've added this across a number of my systems now, and it seems to work
really well (Thank You), especially one system which has a RAID1 with an
external USB drive + internal drive which normally take over 2 days for
a full resync.
It may also be worth reading up on --write-mostly for your external USB
drive, particularly if your workload tends to be single long streaming
reads rather than lots of small parallel ones.
Can you suggest if there is any dis-advantage to using the bitmap (maybe
write performance will suffer), or disk space is reduced, or ....
While I can see the benefits, I'm just wondering if it might be too good
to be true, or what I am missing....
Thanks again for your assistance.
Yes, write performance suffers, especially if you have a small bitmap
chunk size, and the default is as small as possible for the array. See
`mdadm -X /dev/sdX` on one of the components of your array for what the
default was calculated to be for your array, and read the --bitmap-chunk
section of `man mdadm` for a description of bitmap chunk size
considerations. I found a bitmap chunk of 128MB (131072KB) kept write
perfomance very near no-bitmap speeds (both MB/s and IOPS) but kept
resync times fast (a few seconds).
I'm not sure, but you may have to remove the bitmap (--grow
--bitmap=none) before re-adding one with a different chunk size (e.g.
--grow --bitmap=internal --bitmap-chunk=131072).
Cheers,
John.
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