We're doing some testing to determine performance of MD-RAID and suitability for our environment.
One particular test is giving some cause for concern:
- Run heavy I/O to a raw partition:
# time dd if=/dev/zero of=/dev/md0p1 bs=131072 count=1000000
- Run single sync I/Os to the partition:
# time dd if=/dev/zero of=/dev/md0p1 bs=4096 count=1 oflag=sync
When we run this, latency for the single I/O completion can go as high as 5-10 seconds
In investigating this, it looks like the following code in md_write_start causes most of the slow down:
if (mddev->in_sync) {
spin_lock_irq(&mddev->write_lock);
if (mddev->in_sync) {
mddev->in_sync = 0;
set_bit(MD_CHANGE_CLEAN, &mddev->flags);
set_bit(MD_CHANGE_PENDING, &mddev->flags);
md_wakeup_thread(mddev->thread);
did_change = 1;
}
spin_unlock_irq(&mddev->write_lock);
}
When we change this to run about once every 10 seconds, our latency goes way down to a reasonable number of milliseconds.
Questions:
- is the high latency for single sync I/Os something that we should expect?
- the first time the thread runs, it was seen to take a lot longer. Is this due to more outstanding metadata or similar?
- is the approach to run the thread less frequently reasonable, or does that open up huge problems?
Thanks,
Frank
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