[pve-devel] Default cache mode for VM hard drives

[Stanislav German-Evtushenko]

Alexandre,

> do you see the problem with qemu cache=directsync ? (O_DIRECT + O_DSYNC).
Yes, it happens in less number of cases (may be 10 times less) but still
happens. I have a reproducible case with Windows 7 and directsync.

Stanislav

On Thu, May 28, 2015 at 11:18 AM, Alexandre DERUMIER <aderumier at odiso.com>
wrote:

> >>Resume: when working in O_DIRECT mode QEMU has to wait until "write"
> system call is finished before changing this buffer OR QEMU has to create
> new buffer every time OR ... other ideas?
>
> AFAIK, only O_DSYNC can guarantee that data are really written to the last
> layer(disk platters)
>
> That's why we need to use barrier or FUA in last kernel in guest, when
> using O_DIRECT, to be sure that guest filesystem is ok and datas are
> flushed at regular interval.
> (To avoid incoherent filesystem with datas).
>
>
> do you see the problem with qemu cache=directsync ? (O_DIRECT + O_DSYNC).
>
>
>
>
>
> ----- Mail original -----
> De: "Stanislav German-Evtushenko" <ginermail at gmail.com>
> À: "dietmar" <dietmar at proxmox.com>
> Cc: "aderumier" <aderumier at odiso.com>, "pve-devel" <
> pve-devel at pve.proxmox.com>
> Envoyé: Jeudi 28 Mai 2015 09:54:32
> Objet: Re: [pve-devel] Default cache mode for VM hard drives
>
> Dietmar,
>
> fsync esures that data reaches underlying hardware but it does not help
> being sure that buffer is not changed until it is fully written.
>
> I will describe my understanding here why we get this problem with
> O_DIRECT and don't have without.
>
> ** Without O_DIRECT **
> 1. Application tries to write data from buffer
> 2. Data from buffer goes to host cache
> 3. RAID writers get data from host cache and put to /dev/loop1 and
> /dev/loop2
> Even if buffer changes data in host cache will not be changed so RAID is
> consistent.
>
> ** With O_DIRECT **
> 1. Application tries to write data from buffer
> 2. RAID writers get data from application (!!!) bufferand put to
> /dev/loop1 and /dev/loop2
> if meanwhile data in buffer is changed (this change can be done in
> different posix thread) then we have different data reachs /dev/loop1 and
> /dev/loop2
>
> Resume: when working in O_DIRECT mode QEMU has to wait until "write"
> system call is finished before changing this buffer OR QEMU has to create
> new buffer every time OR ... other ideas?
>
> Stanislav
>
> On Thu, May 28, 2015 at 10:31 AM, Dietmar Maurer < dietmar at proxmox.com >
> wrote:
>
>
> > I have just done the same test with mdadm and not DRBD. And what I found
> > that this problem was reproducible on the software raid too, just as it
> was
> > claimed by Lars Ellenberg. It means that problem is not only related to
> > DRBD but to O_DIRECT mode generally when we don't use host cache and a
> > block device reads data directly from userspace.
>
> We simply think the behavior is correct. If you want to be sure data is
> on disk you have to call fsync.
>
>
>
>
>


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