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SubjectRe: Linux and SSD and how to decrease io delay : question, please
Mariella Petrini wrote:
> Hi All,
>
> I have been running Linux kernel 2.6.23 with Debian on
>
> a 2 CPUs Dual-Core AMD Opteron(tm) Processor 2218.
> I have been using an SSD drive (MTRON) connected to a
> raid controller (LSI MegaRAID 8704ELP) to contain
> data stored on several databases.
> Each database contains 20,000 files and there are 50
> databases total (1,000,000 files distributed across 50
> directories on the filesystem on the ssd).
>
> The database server uses the databases very "write"
> intensively.
> An approximate percentage could be 20% reads versus
> 80% random writes.
>
> No matter which filesystem type is used (I have tried
> ext3, xfs, jfs, reiserfs) I see every few seconds
> snapshots (the could last in the worst case up to 200
> seconds, depending on the filesystem type)
> where the iowait on the ssd device goes approximately
> over 50% (up to 100%) and
> the database server threads go to wait (get
> suspended).
>
> Is there any Linux kernel parameter or any Linux
> parameter that could be varied that could help to
> decrease the time spent to sync all the data to the
> SSD device ?
>
I may misunderstand the problem, but have you tried tuning the
parameters /proc/sys/vm/dirty_* to help this? It sounds as if you may be
filling write cache and then flushing. Of course you didn't measure the
disk i/o rates along with this, so it's hard to see what's happening.

--
Bill Davidsen <davidsen@tmr.com>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot


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