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. 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
