Re: [PATCH 0/10] fuse: An attempt to implement a write-back cache policy

[Anand Avati <avati-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Doesn't such a change increase the "dirty debt" held in the kernel and 
therefore increase the probability of resulting in a deadlock (when the 
userspace filesystem's memory allocation request ends up waiting on a 
dirty page writeout caused by this write-back feature)? Why not 
implement the write-back inside the userspace filesystem leaving the 
kernel operate in write-through, which makes things overall less unsafe? 
Do you have performance numbers comparing write-back in kernel v/s 
write-back in userspace?

Avati


On 07/03/2012 08:53 AM, Pavel Emelyanov wrote:
> Hi everyone.
>
> One of the problems with the existing FUSE implementation is that it uses the
> write-through cache policy which results in performance problems on certain
> workloads. E.g. when copying a big file into a FUSE file the cp pushes every
> 128k to the userspace synchronously. This becomes a problem when the userspace
> back-end uses networking for storing the data.
>
> A good solution of this is switching the FUSE page cache into a write-back policy.
> With this file data are pushed to the userspace with big chunks (depending on the
> dirty memory limits, but this is much more than 128k) which lets the FUSE daemons
> handle the size updates in a more efficient manner.
>
> The writeback feature is per-connection and is explicitly configurable at the
> init stage (is it worth making it CAP_SOMETHING protected?) When the writeback is
> turned ON:
>
> * still copy writeback pages to temporary buffer when sending a writeback request
>    and finish the page writeback immediately
>
> * make kernel maintain the inode's i_size to avoid frequent i_size synchronization
>    with the user space
>
> * take NR_WRITEBACK_TEMP into account when makeing balance_dirty_pages decision.
>    This protects us from having too many dirty pages on FUSE
>
> The provided patchset survives the fsx test. Performance measurements are not yet
> all finished, but the mentioned copying of a huge file becomes noticeably faster
> even on machines with few RAM and doesn't make the system stuck (the dirty pages
> balancer does its work OK). Applies on top of v3.5-rc4.
>
> We are currently exploring this with our own distributed storage implementation
> which is heavily oriented on storing big blobs of data with extremely rare meta-data
> updates (virtual machines' and containers' disk images). With the existing cache
> policy a typical usage scenario -- copying a big VM disk into a cloud -- takes way
> too much time to proceed, much longer than if it was simply scp-ed over the same
> network. The write-back policy (as I mentioned) noticeably improves this scenario.
> Kirill (in Cc) can share more details about the performance and the storage concepts
> details if required.
>
> Thanks,
> Pavel
>
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