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From Vladimir Ozerov <voze...@gridgain.com>
Subject Re: IGFS concurrency issue
Date Wed, 07 Oct 2015 06:11:38 GMT
Cos,
Yes, no long-time locking is expected here.

On Wed, Oct 7, 2015 at 6:57 AM, Konstantin Boudnik <cos@apache.org> wrote:

> IIRC NN should be locking on these ops anyway, shouldn't it? The situation
> is
> no different if multiple clients are doing these operations
> near-simultaneously. Unless I missed something here...
>
> On Thu, Sep 24, 2015 at 11:28AM, Sergi Vladykin wrote:
> > May be just check that they are not parent-child within the tx?
> >
> > Sergi
> > Igniters,
> >
> > We revealed concurrency problem in IGFS and I would like to discuss
> > possible solutions to it.
> >
> > Consider the following file system structure:
> > root
> > |-- A
> > |   |-- B
> > |   |   |-- C
> > |   |-- D
> >
> > ... two concurrent operations in different threads:
> > T1: move(/A/B, /A/D);
> > T2: move(/A/D, /A/B/C);
> >
> > ... and how IGFS handles it now:
> > T1: verify that "/A/B" and "/A/D" exist, they are not child-parent to
> each
> > other, etc. -> OK.
> > T2: do the same for "A/D" and "A/B/C" -> OK.
> > T1: get IDs of "/A", "/A/B" and "/A/D" to lock them later inside tx.
> > T2: get IDs of "/A", "/A/D", "/A/B" and "/A/B/C" to lock them later
> inside
> > tx.
> >
> > T1: Start pessimistic tx, lock IDs of "/A", "/A/B", "/A/D", perform move
> ->
> > OK.
> > root
> > |-- A
> > |   |-- D
> > |   |   |-- B
> > |   |   |   |-- C
> >
> > T2: Start pessimistic tx, lock IDs of "/A", "/A/D", "/A/B" and
> > "/A/B/C" (*directory
> > structure already changed at this time!*), perform move -> OK.
> > root
> > |-- A
> > B
> > |-- D
> > |   |-- C
> > |   |   |-- B (loop!)
> >
> > File system is corrupted. Folders B, C and D are not reacheable from
> root.
> >
> > To fix this now we additionaly check if directory structure is still
> > valid *inside
> > transaction*. It works, no more corruptions. But it requres taking locks
> on
> > the whole paths *including root*. So move, delete and mkdirs opeartions
> *can
> > no longer be concurrent*.
> >
> > Probably there is a way to relax this while still ensuring consistency,
> but
> > I do not see how. One idea is to store real path inside each entry. This
> > way we will be able to ensure that it is still at a valid location
> without
> > blocking parents, so concurrnecy will be restored. But we will have to
> > propagate strucutral changes to children. E.g. move of a folder with 100
> > items will lead to update of >100 cache entries. Not so good.
> >
> > Any other ideas?
> >
> > Vladimir.
>

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