>From: matthew patton [mailto:patto...@yahoo.com] >Sent: Tuesday, April 20, 2010 12:54 PM > >Geoff Nordli <geo...@grokworx.com> wrote: > >> With our particular use case we are going to do a "save >> state" on their >> virtual machines, which is going to write 100-400 MB >> per VM via CIFS or >> NFS, then we take a snapshot of the volume, which >> guarantees we get a >> consistent copy of their VM. > >maybe you left out a detail or two but I can't see how your ZFS snapshot >is going to be consistent UNLESS every VM on that ZFS volume is >prevented from doing any and all I/O from the time it finishes "save >state" and you take your ZFS snapshot. > >If by "save state" you mean something akin to VMWare's disk snapshot, >why would you even bother with a ZFS snapshot in addition? >
We are using VirtualBox as our hypervisor. When it does a save state it generates a memory file. The memory file plus the volume snapshot creates a consistent state. In our platform each student's VM points to a unique backend volume via iscsi using VBox's built-in iscsi initiator. So there is a one-to-one relationship between VM and Volume. Just for clarity, a single VM could have multiple disks attached to it. In that scenario, then a VM would have multiple volumes. >> end we could have >> maybe 20-30 VMs getting saved at the same time, which could >> mean several GB >> of data would need to get written in a short time frame and >> would need to >> get committed to disk. >> >> So it seems the best case would be to get those "save >> state" writes as sync >> and get them into a ZIL. > >That I/O pattern is vastly >32kb and so will hit the 'rust' ZIL (which >ALWAYS exists) and if you were thinking an SSD would help you, I don't >see any/much evidence it will buy you anything. > > If I set the logbias (b122) to latency, then it will direct all sync IO to the log device, even if it exceeds the zfs_immediate_write_sz threshold. Have great day! Geoff _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
