----- 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. Thanks Manoj Nayak > For writes, you should see them become coalesced, so rather than > sending 3 42.6kByte write requests to a vdev, you might see one > 128kByte write request. > > In other words, ZFS has an I/O scheduler which is responsible > for sending I/O requests to vdevs. > -- richard > > _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
