Adam Goryachev schrieb:
>> Having battery backed RAM on the RAID controller can help, because the
>> controller can lie to the OS and say the data is written to disk
>> immediately instead of waiting for an read-calculate-write cycle,
>> since it's sure that if it does lose power, it can store the data that
>> should be written to disk later when power is restored in addition to
>> buffering the reads/writes so that it can reorder them to reduce the
>> amount of seeking required.
>
> Is it possible to instruct linux to use it's memory to do this? If you
> have a UPS and feel that it is pretty unlikely to crash, you might be
> happy to get this kind of speed improvement.
To some extent, it is.
Do a "cat /proc/meminfo", and look at "Dirty". Basically, this is what
is still in memory, and should/will go to the disk, soon.
For a 1.2 TB filesystem (~65% full) storing about 14 TB of data ("prior
to pooling and compression"), I use these values to tune this behaviour
(they are documented in Documentation/filesystems/proc.txt):
echo 50 > /proc/sys/vm/dirty_ratio
echo 50 > /proc/sys/vm/dirty_background_ratio
echo 6000 > /proc/sys/vm/dirty_writeback_centisecs
echo 6000 > /proc/sys/vm/dirty_expire_centisecs
That's with 2.7 GB RAM.
Right now, I have almost 0.5 GB of "Dirty" memory.
# cat /proc/meminfo
(...)
Dirty: 428172 kB
Writeback: 868 kB
Issuing a "sync" would cause a major slowdown for some time - with
enough RAM and some tuning, most writes can be scheduled for later and
can happen when it's optimal. Of course, sooner or later you will be out
of your RAM anyway.
I also use a "commit=60" mount option to ext3.
I found the best results with anticipatory IO scheduler
(Documentation/block/as-iosched.txt), which tries to reduce seeks.
--
Tomasz Chmielewski
http://wpkg.org
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