On Sunday 15 June 2014 at 23:49:59, Edward Shishkin wrote:
>
> On 06/15/2014 08:07 PM, Ivan Shapovalov wrote:
> > On Sunday 15 June 2014 at 19:36:05, Edward Shishkin wrote:
> >> On 06/13/2014 10:28 PM, Ivan Shapovalov wrote:
> >>> Here is my "analysis" of what happens in reiser4 during a transaction's
> >>> lifetime wrt. block allocation and deallocation.
> >>>
> >>>
> >>> THE EFFECTS (SEMANTICS) OF RELATED FUNCTIONS
> >>> reiser4_alloc_blocks_bitmap(): allocates in WORKING BITMAP
> >> Yes.
> >>
> >>> reiser4_dealloc_blocks_bitmap(!BA_DEFER): deallocates from WORKING BITMAP
> >>> reiser4_dealloc_blocks_bitmap(BA_DEFER): stores to ->delete_set
> >> This is correct for the middle-level allocator (without suffix "bitmap").
> >> The low-level one frees blocks only in WORKING BITMAP.
> > Yes, this was a wording mistake. I've noticed it shortly after sending...
> >
> >>> reiser4_pre_commit_hook_bitmap(): allocates all relocated nodes in COMMIT BITMAP
> >> "Relocated" is bad term here: nodes with new data also get the flag
> >> JNODE_RELOC. So, I would rather say, that it applies freshly allocated
> >> nodes of the atom to COMMIT BITMAP.
> >>
> >>
> >>> deallocates ->delete_set from COMMIT BITMAP
> >> applies the atom's delete_set to COMMIT BITMAP
> > I've meant exactly this.
> >
> >>> reiser4_post_commit_hook(): deallocates ->delete_set using !BA_DEFER
> >>> (i. e. from WORKING BITMAP)
> >> applies the atom's delete_set to WORKING BITMAP.
> > ditto
> >
> >> I would also mention a function of the middle-level block allocator
> >> reiser4_alloc_blocks(): allocates blocks in WORKING BITMAP.
> > Yes, sure.
> >
> >
> >> Note that the middle-level block allocator (block_alloc.c) actually
> >> manipulates
> >> with abstract space maps. Currently in reiser4 they are represented only by
> >> bitmaps (plugin/space/bitmap.c). We can also implement another
> >> representation -
> >> extent tree (like in XFS). I don't see any needs for now, though.
> > Extent trees seem to be interesting.. They claim that they are more efficient -
> > is the difference that huge?
> >
> >>> TIMELINE OF ALLOCATIONS FOR "USUAL" NODES, AND TIMELINE OF TRANSACTION COMMIT
> >>> - nodes are allocated using reiser4_alloc_blocks() and setting JNODE_RELOC,
> >>> so WORKING BITMAP ensures that two nodes cannot get the same block;
> >>> - nodes are deallocated using reiser4_dealloc_blocks(BA_DEFER),
> >>> so their deallocation is not immediately reflected in WORKING BITMAP;
> >>> (the relocate set is written here)
> >>> - reiser4_pre_commit_hook_bitmap() uses 1) JNODE_RELOC flag and 2) ->delete_set
> >>> to convey effective bitmap changes into COMMIT BITMAP;
> >>> (the journal and overwrite set are written here)
> >>> - reiser4_post_commit_hook() uses ->delete_set to convey deallocations
> >>> from step 2 to WORKING BITMAP.
> >>> (the discard happens here)
> >>>
> >>>
> >>> TIMELINE OF ALLOCATIONS FOR WANDERED JOURNAL BLOCKS
> >>> - at commit time, blocks are allocated using reiser4_alloc_blocks(), so they
> >>> are allocated in WORKING BITMAP and do not interfere with any "usual" blocks;
> >>> - after writing wandered blocks, they are deallocated using
> >>> reiser4_dealloc_blocks(!BA_DEFER), i. e. from the WORKING BITMAP.
> >>
> >> So, the system of working and commit bitmaps plus the delete set seems
> >> to be redundant? I think this is because of performance reasons: block
> >> allocation is critical thing...
> > Seems like it is -- for performance and simplicity (== robustness).
> >
> >>> CONCLUSION
> >>> At possible transaction replay time, journal blocks are not allocated in any
> >>> of the bitmaps. However, because the journal is read and replayed before a
> >>> transaction has a chance to commit, this fact does not matter.
> >>> What matters is that wandered journal blocks never hit COMMIT BITMAP.
> >>>
> >>> So, if I've got all this correct (which I highly doubt), the disk space leak
> >>> (as you pointed it out) does not exist.
> >> It seems, you are right..
> >>
> >>> What exists is a rather different problem with my idea of "log every
> >>> deallocated block". Current implementation logs every block regardless of
> >>> BA_DEFER flag presence or absence, so non-wandered blocks are logged twice.
> >>>
> >>> We could just use ->delete_set, but we would lose wandered blocks then.
> >>> Or we could only log !BA_DEFER requests, which would do the right thing
> >>> (wandered blocks + deallocations from reiser4_post_commit_hook()), but
> >>> the reasoning behind this decision would not be obvious for a casual
> >>> code reader.
> >> I think that a good comment will save the situation..
> >>
> >>> Or we could log only wandered blocks (in addition to ->delete_set)
> >>> at discard time, but this is messy and requires us to merge the discard log
> >>> with ->delete_set at discard time.
> >> what is the difference with the previous "we could.."?
> > The previous option is to log all !BA_DEFER requests in addition to
> > ->delete_set, so those two block sets would be partially overlapping.
>
>
> Hmm.. Are they really overlapped?
>
> reiser4_dealloc_blocks() looks like the following:
> {
> if (flags & BA_DEFER)
> log in the delete_set;
> else
> deallocate in working bitmap
> ...
> }
>
> so (!BA_DEFER) requests and the delete_set look disjoint,
> or, I missed something?
You're right, I've misread the code. I was under impression that
reiser4_post_commit_hook() internally calls reiser4_dealloc_blocks(!BA_DEFER).
So the second option (which I prefer) is to log all sa_dealloc_blocks() calls.
That is, effectively, ->delete_set plus wandered journal block deallocations.
> > And this option is to log only those !BA_DEFER requests which are not in
> > ->delete_set, so no block will be stored twice. Thus we will have to consider
> > both ->delete_set and this hypothetical ->discard_set at discard time.
> >
> >>> Or we could log wandered blocks straight into ->delete_set and do something in
> >>> reiser4_post_commit_hook() to separate these entries, but this is super messy.
> >>>
> >>> I'm preferring the second way... Edward, please proof-read all this.
> >> BTW, what about your current implementation? Does it work for you?
> > Yes, it apparently does (no massive corruptions).
> >
> > I get batches of "orphan cluster" errors from time to time (from offset 0 till
> > the supposed end of file),
>
>
> this is fsck complaints?
Yes. (I do `fsck.reiser4 --build-fs` pretty routinely...)
> > but 1) I do not see which files get corrupted,
> > 2) this could be due to txmod=wa, 3) my machine is pretty crashy now due to
> > some driver issues, so I don't know what exactly causes this.
> >
> > Thanks for proof-reading. What do you think about all this?
>
>
> I think that not bad for the beginning! I have purchased an SSD card,
> hope to test the discard stuff in the near future.
And I hope to finish this feature somewhen... :)
Thanks,
--
Ivan Shapovalov / intelfx /
>
> Thanks,
> Edward.
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