[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [X-POST from xen-users] DomU Sync I/O
Good morning everyone, while trying out xenserver there was a thing that caught my attention: apparently every I/O done in the guest magically ends being async toward the storage (amongst other things I tried dd with oflags=sync). The storage BOX exports the LUNs via FC, with writeback cache enabled. The very same dd command issued from dom0, targeted to the same storage (onto an LV manually created on the SR VG) acts correctly, i.e. I see SCSI command 0x35 (SYNCHRONIZE CACHE) issued to the target after every single block (confirmed via quick-n-dirty dtrace script, storage is a zfs-based appliance); that doesn't happen for the I/O issued from the guest. In my point of view, this is a big issue which may compromise data integrity in the event of a power failure or a generic HW fault in the storage BOX, which may lead to guest filesystems corruption due to the possibility of journal becoming corrupt. As a workaround I can obviously disable writeback cache on the target, but that leads to unwarranted performance loss (because every single I/O becomes synchronous, even there's no need to). Am I missing something ? Some hypothesis I made: - tapblk3 drops sync cache commands from the guest - xen_blkfront into the domU ignores sync I/O semantics I'm running version 6.5.0-90233c. Any thoughts ? BR, Simone. P.S: I already tried writing in the appropriate forum (the xenserver one), but without luckâIâm feeling a bit hopeless. P.P.S: My apologize for the cross posting, but again Iâm clueless. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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