Hi,
I've been investigating some performance issues with writes to a LVM2
snapshot origin volume being much slower than one would expect (i.e.,
10-50x slower for a logical volume with a single snapshot volume).
Using a simple dd test (I get similar numbers with other tools like
bonnie) on a P4 3GHz w/SATA disks (no RAID):
dd if=/dev/zero of=/vol/file bs=1M count=1000
Logical volume w/o snapshot: 45.4 MB/s
LV w/snapshot on same disk, chunksize=8K: 1.1 MB/s
LV w/snapshot on same disk, chunksize=512K: 4.0 MB/s
LV w/snapshot on separate disk, chunksize=8K: 1.2 MB/s
LV w/snapshot on separate disk, chunksize=512K: 5.6 MB/s
I tracked the main cause of the poor performance to the behaviour of
process_jobs() in kcopyd. If kcopyd's page pool has no free pages, it
moves the next job in the _pages_jobs list to the tail of the list.
New jobs are also added to the tail of the _pages_jobs list (in
dispatch_job()). This re-ordering of the I/Os results in a lot of extra
seek activity for workloads consisting of sequential writes.
The below patch modifies process_jobs() to push the job back to the head
of the list instead of the tail when there are no free pages. I see
significantly improved performance with this change:
LV w/snapshot on same disk, chunksize=8K: 9.6 MB/s
LV w/snapshot on same disk, chunksize=512K: 8.4 MB/s
LV w/snapshot on separate disk, chunksize=8K: 17.2 MB/s
LV w/snapshot on separate disk, chunksize=512K: 14.5 MB/s
Thanks,
Kevin
--- linux/drivers/md/kcopyd.c.bak 2007-05-30 14:18:36.000000000 -0700
+++ linux/drivers/md/kcopyd.c 2007-05-30 15:54:33.000000000 -0700
@@ -280,6 +280,15 @@
spin_unlock_irqrestore(&_job_lock, flags);
}
+static inline void push_head(struct list_head *jobs, struct kcopyd_job *job)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&_job_lock, flags);
+ list_add(&job->list, jobs);
+ spin_unlock_irqrestore(&_job_lock, flags);
+}
+
/*
* These three functions process 1 item from the corresponding
* job list.
@@ -404,7 +413,7 @@
* We couldn't service this job ATM, so
* push this job back onto the list.
*/
- push(jobs, job);
+ push_head(jobs, job);
break;
}
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