[sheepdog] [PATCH v2 0/5] garbage collect needless VIDs and inode objects

Mon Mar 16 14:19:49 CET 2015

On Mon, Mar 16, 2015 at 09:37:45PM +0900, Hitoshi Mitake wrote:
> On Mon, Mar 16, 2015 at 4:39 PM, Liu Yuan <namei.unix at gmail.com> wrote:
> > On Mon, Mar 16, 2015 at 03:55:43PM +0900, Hitoshi Mitake wrote:
> >> At Mon, 16 Mar 2015 14:48:47 +0800,
> >> Liu Yuan wrote:
> >> >
> >> > On Mon, Mar 16, 2015 at 03:39:17PM +0900, Hitoshi Mitake wrote:
> >> > > At Mon, 16 Mar 2015 14:31:44 +0800,
> >> > > Liu Yuan wrote:
> >> > > >
> >> > > > On Mon, Mar 16, 2015 at 02:08:25PM +0800, Liu Yuan wrote:
> >> > > > > On Mon, Mar 16, 2015 at 02:36:57PM +0900, Hitoshi Mitake wrote:
> >> > > > > > At Mon, 16 Mar 2015 10:21:50 +0800,
> >> > > > > > Liu Yuan wrote:
> >> > > > > > >
> >> > > > > > > On Thu, Mar 12, 2015 at 08:14:33PM +0900, Hitoshi Mitake wrote:
> >> > > > > > > > At Thu, 12 Mar 2015 14:41:56 +0800,
> >> > > > > > > > Liu Yuan wrote:
> >> > > > > > > > >
> >> > > > > > > > > On Tue, Jan 13, 2015 at 10:37:40AM +0900, Hitoshi Mitake wrote:
> >> > > > > > > > > > Current sheepdog never recycles VIDs. But it will cause problems
> >> > > > > > > > > > e.g. VID space exhaustion, too much garbage inode objects.
> >> > > > > > > > > >
> >> > > > > > > > > > Keeping deleted inode objects is required because living inodes
> >> > > > > > > > > > (snapshots or clones) can point objects of the deleted inodes. So if
> >> > > > > > > > > > every member of VDI family is deleted, it is safe to remove deleted
> >> > > > > > > > > > inode objects.
> >> > > > > > > > > >
> >> > > > > > > > > > v2:
> >> > > > > > > > > >  - update test scripts
> >> > > > > > > > >
> >> > > > > > > > > All the nodes of our test cluster panic out for the following problem:
> >> > > > > > > > >
> >> > > > > > > > > Mar 12 00:05:03  DEBUG [main] zk_handle_notify(1216) NOTIFY
> >> > > > > > > > > Mar 12 00:05:03  DEBUG [main] sd_notify_handler(960) op NOTIFY_VDI_ADD, size: 96, from: IPv4 ip:192.168.39.177 port:7000
> >> > > > > > > > > Mar 12 00:05:03  DEBUG [main] do_add_vdi_state(362) 7c2b2b, 3, 0, 22, 0
> >> > > > > > > > > Mar 12 00:05:03  DEBUG [main] do_add_vdi_state(362) 7c2b2c, 3, 0, 22, 7c2b2b
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] update_vdi_family(127) PANIC: parent VID: 7c2b2b not found
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] crash_handler(286) sheep exits unexpectedly (Aborted), si pid 4786, uid 0, errno 0, code -6
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(833) sheep.c:288: crash_handler
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(847) /lib64/libpthread.so.0() [0x338200f4ff]
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(847) /lib64/libc.so.6(gsignal+0x34) [0x3381c328a4]
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(847) /lib64/libc.so.6(abort+0x174) [0x3381c34084]
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(833) vdi.c:127: update_vdi_family
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(833) vdi.c:398: add_vdi_state
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(833) ops.c:711: cluster_notify_vdi_add
> >> > > > > > > > > Mar 12 00:05:03  EMERG [main] sd_backtrace(833) group.c:975: sd_notify_handler
> >> > > > > > > > >
> >> > > > > > > > > So I tracked back to this patch set. The problem of this patch set tried to
> >> > > > > > > > > solve is very clear and come along with sheepdog since its born. This reveals
> >> > > > > > > > > actually the defeciency of our vdi allocation algorithm, which we need rethink
> >> > > > > > > > > a completely new algorithm to replace it and is not fixable, unfortunately.
> >> > > > > > > > >
> >> > > > > > > > > One simple rule, we can't recyle any vid if it is once created because of its
> >> > > > > > > > > current hash collision handling. Our current implementation forbigs recycling.
> >> > > > > > > > >
> >> > > > > > > > > Instead of fixing the above panic bug, I'd suggest we revert this patch set.
> >> > > > > > > > > For the problem this patch set mentioned, I think we need a new algoirthm and
> >> > > > > > > > > implementation. But before that, we should stay with old one, it is stable and
> >> > > > > > > > > reliable and should work for small size cluster.
> >> > > > > > > > >
> >> > > > > > > > > How do you think, Hitoshi and Kazutaka?
> >> > > > > > > >
> >> > > > > > > > How about providing switch turn on/off VID recycling? e.g. dog cluster
> >> > > > > > > > format --enable-vid-recycle. The code can easily be pushed into
> >> > > > > > > > conditional branches. I can post a patch if this way is good for you.
> >> > > > > > > >
> >> > > > > > >
> >> > > > > > > This temporary workaroud looks okay but not good enough to me, what I am
> >> > > > > > > concerned is that vdi recycle will probably never be implemented if we stick to
> >> > > > > > > current vdi allocation algorithm. Once the new vdi allocation is intruduced
> >> > > > > > > someday in the future, the new algorithm would have no this kind of problem at
> >> > > > > > > all. If this is the case, the above code we leave here is also useless.
> >> > > > > > >
> >> > > > > > > I think we should focus on the new vdi allocation algorithm, e.g, store
> >> > > > > > > {name, vid} directly into a kv engine either implemented by sheep or by with the
> >> > > > > > > help of other software like zookeeper.
> >> > > > > > >
> >> > > > > > > I'm inclined to revert above patch set, for
> >> > > > > > > 1. it can't fix a non-fixable problem inherently
> >> > > > > > > 2. the code is probalematic and can cause a catastraphic disaster (all node die)
> >> > > > > > > 3. we might not need it in the future because it is specific for current vdi
> >> > > > > > >    allocation algorithm.
> >> > > > > >
> >> > > > > > We can simply employ whole range lookup of bitmap as a VID allocation
> >> > > > > > algorithm for recycling policy. Of course it would be harmful for
> >> > > > > > snapshot and clone creation, but it can work correctly (and we have
> >> > > > > > optimization e.g. parallelizing, caching, etc). In addition, the
> >> > > > > > performance degradation can happen potentially even if we use the
> >> > > > > > existing VID allocation algorithm (e.g. hash collision, although
> >> > > > > > course it can happen rearely).
> >> > > > >
> >> > > > > Do we really need vdi recycle if we bring very complex lines of code? Current
> >> > > > > algorithm can *reuse* of deleted vdi IDs and inodes. So the very problem is
> >> > > > > actually the space effeciency, so you try to reclaim the space occupied by
> >> > > > > deleted vdis.
> >> > > > >
> >> > > > > If it is very easy to reclaim deleted inodes, I'd say great and let's go ahead.
> >> > > > > But it apparently not. We have this patch set and then the lookup algorithm is
> >> > > > > heavily degrated.
> >> > > > >
> >> > > > > I'm afraid, lookup the whole range is too costy, considering the deleted inodes
> >> > > > > space we reclaim. I think, most of users can bear the very little space overhead
> >> > > > > for better performance. So this patch set trade the code complexity and
> >> > > > > performance for the space efficiency. Note, we can reclaim inodes only in the
> >> > > > > case that we delete the whole snapshot chain and parent. This is actually a rare
> >> > > > > case.
> >> > > >
> >> > > > Delete the vdi & snapthos data objects is really good enough to me. Your patch
> >> > > > set is the one of efforts to perfect current algorithms. But the cost is too
> >> > > > high because the hottest path of vdi_lookup() is heavily degrated for gerenal
> >> > > > cases, even though later we can fix all the bugs related to the this patch set.
> >> > > >
> >> > > > Please consider it.
> >> > >
> >> > > Of course the increased cost of vdi_lookup() is problem. So I'm
> >> > > posting a patch for providing an option for enabling/disabling vid
> >> > > recycling. In default, the recycling will be disabled with the
> >> > > patch. So users can choose two different policies with different
> >> > > pros/cons.
> >> > >
> >> > > The recycling VID is an actual requirement from the development team
> >> > > of NTT DATA. I need to provide it at least as an option.
> >> >
> >> > THe requrement is for reclaim the deleted inodes? The vid exaustion problem
> >> > mentioned in your patch set, is actually not a problem, no? We can reuse deleted
> >> > inode and vid. If so, reclaim the deleted inodes, which are very little and
> >> > reuable, is so important?
> >>
> >> The primary problem is VID. But reusing deleted VID correctly will
> >> require mucm more complex code e.g. rewriting parent/child
> >> relationship (and it would be superset of current code).
> >
> > I guess so. I don't have a glance of this problem in a code manner, but we have
> > two state to indicate if the vid is used or not,
> >
> > 1. bit int bitmap of system_info
> > 2. inode's name field
> >
> > For the non-snapshot case, the valid vid is a) bit is set, b) name is not empty.
> > So if we zero name, we can resue this bit by check if name is empty.
> >
> > For the snapshot case, the above criteria still hold true, no?
> >
> > 001111000
> >   ABCD
> >
> > D is the working vid, ABC are the snapshots. After we remove B,
> >
> > 0011 11000
> >   AB'CD
> >
> > B' mean its name is empty. So the snapshot chain becomes A->C->D. B is still
> > reusable, no?
> >
> 
> No. Because C and D can point objects created when B was a working VDI.

You are right, but your patch set won't fix this problem either. I am not
concerned this problem, but rather:

1. code complexity and code stability.
2. heavily degraded vdi_lookup().

If we fix one problem by introducing yet another problem(2), I'd suggest we
think twice.

Yuan