[sheepdog] [Qemu-devel] [PATCH] sheepdog: fix overlapping metadata update

[Jeff Cody]

On Thu, Jul 30, 2015 at 09:41:08AM +0300, Vasiliy Tolstov wrote:
> 2015-07-29 12:31 GMT+03:00 Liu Yuan <namei.unix at gmail.com>:
> > Technically, it won't affect the performance because index updates are not range
> > but concrete in terms of underlying 4M block size. Only 2 or 3 indexes in a
> > range will be updated and 90+% updates will be only 1. So if 2 updates stride a
> > large range, it will actually worse the performance of sheepdog because many
> > additional unref of object will be executed by sheep internally.
> >
> > It is not a performance problem but more the right fix. Even with your patch,
> > updates of inode can overlap. You just don't allow overlapped requests go to
> > sheepdog, which is a overkill approach. IMHO, we should only adjust to avoid
> > the overlapped inode updates, which can be done easily and incrementally on top
> > of old code, rather than take on a complete new untested overkill mechanism. So
> > what we get from your patch? Covering the problem and lock every requests?
> >
> > Your patch actually fix nothing but just cover the problem by slowing down the
> > request and even with your patch, the problem still exists because inode updates
> > can overlap. Your commit log doesn't explain what is the real problem and why
> > your fix works. This is not your toy project that can commit whatever you want.
> >
> >> BTW, sheepdog project was already forked, why don't you fork the block
> >> driver, too?
> >
> > What makes you think you own the block driver?
> >
> > We forked the sheepdog project because it is low quality of code partly and mostly
> > some company tries to make it a private project. It is not as open source friendly
> > as before and that is the main reason Kazutaka and I chose to fork the sheepdog
> > project. But this doesn't mean we need to fork the QEMU project, it is an
> > open source project and not your home toy.
> >
> > Kazutaka and I are the biggest contributers of both sheepdog and QEMU sheepdog
> > block driver for years, so I think I am eligible to review the patch and
> > responsible to suggest the right fix. If you are pissed off when someone else
> > have other opinions, you can just fork the code and play with it at home or you
> > follow the rule of open source project.
> 
> 
> Jeff Cody, please be the judge, patch from Hitoshi solved my problem
> that i emailed in sheepdog list (i have test environment with 8 hosts
> on each 6 SSD disks and infiniband interconnect between hosts) before
> Hitoshi patch, massive writing to sheepdog storage breaks file system
> and corrupt it.
> After the patch i don't see issues.
>

I'd rather see some sort consensus amongst Liu, Hitoshi, yourself, or
others more intimately familiar with sheepdog.

Right now, we have Hitoshi's patch in the main git repo, slated for
2.4 release (which is Monday).  It sounds, from Liu's email, as this
may not fix the root cause.

Vasiliy said he would test Liu's patch; if he can confirm this new
patch fix, then I would be inclined to use Liu's patch, based on the
detailed analysis of the issue in the commit message.

Thanks,
Jeff