manoj nayak wrote: > > ----- Original Message ----- From: "Richard Elling" > <[EMAIL PROTECTED]> > To: "manoj nayak" <[EMAIL PROTECTED]> > Cc: <zfs-discuss@opensolaris.org> > Sent: Wednesday, January 23, 2008 7:20 AM > Subject: Re: [zfs-discuss] ZFS vq_max_pending value ? > > >> manoj nayak wrote: >>> >>>> Manoj Nayak wrote: >>>>> Hi All. >>>>> >>>>> ZFS document says ZFS schedules it's I/O in such way that it >>>>> manages to saturate a single disk bandwidth using enough >>>>> concurrent 128K I/O. >>>>> The no of concurrent I/O is decided by vq_max_pending.The default >>>>> value for vq_max_pending is 35. >>>>> >>>>> We have created 4-disk raid-z group inside ZFS pool on Thumper.ZFS >>>>> record size is set to 128k.When we read/write a 128K record ,it >>>>> issue a >>>>> 128K/3 I/O to each of the 3 data disks in the 4-disk raid-z group. >>>>> >>>> >>>> Yes, this is how it works for a read without errors. For a write, you >>>> should see 4 writes, each 128KBytes/3. Writes may also be >>>> coalesced, so you may see larger physical writes. >>>> >>>>> We need to saturate all three data disk bandwidth in the Raidz >>>>> group.Is it required to set vq_max_pending value to 35*3=135 ? >>>>> >>>> >>>> No. vq_max_pending applies to each vdev. >>> >>> 4 disk raidz group issues 128k/3=42.6k io to each individual data >>> disk.If 35 concurrent 128k IO is enough to saturate a disk( vdev ) , >>> then 35*3=105 concurrent 42k io will be required to saturates the >>> same disk. >> >> ZFS doesn't know anything about disk saturation. It will send >> up to vq_max_pending I/O requests per vdev (usually a vdev is a >> disk). It will try to keep vq_max_pending I/O requests queued to >> the vdev. > > I can see the "avg pending I/Os" hitting my vq_max_pending limit, > then raising the limit would be a good thing. I think , it's due to > many 42k Read IO to individual disk in the 4 disk raidz group.
You're dealing with a queue here. iostat's average pending I/Os represents the queue depth. Some devices can't handle a large queue. In any case, queuing theory applies. Note that for reads, the disk will likely have a track cache, so it is not a good assumption that a read I/O will require a media access. -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
