Greetings Ryusuke,
I'm floating on air at the moment, as I have a working solution.
Turns out Stephene Chazelas had submitted bug at:
https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/595117
AND a small patch to 'qemu-nbd.c', which I applied and tested with
great success. Since I'm only using the loop filesystem aspect of qemu,
I couldn't use the 'cache=writeback' with the qemu VM tools, as you suggested.
However, with the patch, I can submit the same logic to 'qemu-nbd
--cache=writeback ..'
Prior to the patch I would get >10k checkpoints by copying ~1/2gigbyte data
to the loop device -- now I get 2-5 checkpoints -- I'm so stoked!!
Thank you for your kind attention.
NILFS rocks!!
Regards,
Mark T.
-----Original Message-----
From: Ryusuke Konishi [mailto:[email protected]]
Sent: Wednesday, June 27, 2012 2:14 PM
To: Mark Trumpold
Cc: [email protected]; [email protected]
Subject: Re: nilfs checkpoints tied to synchronous writes
>On Mon, 2012-06-25 at 18:11 -0700, Mark Trumpold wrote:
> Hello,
>
> I have a configuration where I use qemu/qcow loop filesystems on top a
> nilfs filesystem to yield writeable snapshots.
>
> The symptom I see is the nilfs filesystem "ballooning" in size when
> writing large chunks of data to the qcow loop filesystem. I also see
> _many_ nilfs checkpoints with small "NBLKINC" counts (>20 <50).
>
> My working theory is nilfs creates checkpoints for each synchronous
> write, and qcow insists on writing synchronously, thus the ballooning
> of checkpoints.
Hi Mark, thanks for your information.
Did you try cache=writeback option on qemu ?
Qemu looks to be using synchronous write operations heavily in its default
cache mode. It's too expensive for underlying filesystems especially for nilfs.
As you pointed out, synchronous write can be separated from checkpointing in
theory. And, nilfs is partially applying such optimization by default (i.e.
"order=relaxed" option). But the current optimization is not sufficient.
I am actually thinking nilfs should have enhanced functionalilty which stops
making checkpoints on data integrity operations like fsync or fdatasync.
Unfortunately, it needs disk format extension to make it possible to recover
data blocks written by fsync/fdatasync after unexpected shutdown/reboot.
Garbage collector also has to handle such extension.
So, these are one of todo items at present.
Thanks,
Ryusuke Konishi
> On 6/26/12 11:16 PM, "Vyacheslav Dubeyko" <[email protected]> wrote:
>
> >Hi Mark,
> >
> >On Mon, 2012-06-25 at 18:11 -0700, Mark Trumpold wrote:
> >> Hello,
> >>
> >> I have a configuration where I use qemu/qcow loop filesystems on
> >> top a nilfs filesystem to yield writeable snapshots.
> >>
> >> The symptom I see is the nilfs filesystem "ballooning" in size when
> >> writing large chunks of data to the qcow loop filesystem. I also
> >> see _many_ nilfs checkpoints with small "NBLKINC" counts (>20 <50).
> >>
> >> My working theory is nilfs creates checkpoints for each synchronous
> >> write, and qcow insists on writing synchronously, thus the
> >> ballooning of checkpoints.
> >
> >I can cite man page for NILFS2:
> >"When data is written or any change is made to a NILFS2 file system,
> >it automatically creates a checkpoint. A checkpoint represents a
> >consistent state of the NILFS2 file system of a certain instant.".
> >
> >>
> >> One question is would it be possible to have nilfs not checkpoint
> >> so often. I've tried the 'nilfs-tune -i ..' to no avail -
> >> apparently synchronous writes overrule this setting.
> >
> >As you can see, checkpoint is a base technique of NILFS2 functioning.
> >But you can manipulate by checkpoints during NILFS2 functioning by
> >means of nilfs-cleanerd daemon.
> >
> >Do you use nilfs-cleanerd daemon?
> >
> >Please, read man page for nilfs-cleanerd daemon and its configuration
> >file. I can cite man page:
> >"nilfs_cleanerd is a system daemon which reclaims disk space of
> >NILFS2 file system found in device."
> >
> >With the best regards,
> >Vyacheslav Dubeyko.
> >
>
>
>
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