Date:	Mon, 11 Oct 2010 14:59:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	"Ted Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH RFC 0/3] Block reservation for ext3

On Mon, 11 Oct 2010 16:28:13 +0200
Jan Kara <jack@...e.cz> wrote:

> On Sat 09-10-10 14:03:58, Ted Ts'o wrote:
> > On Sat, Oct 09, 2010 at 02:12:24AM +0200, Jan Kara wrote:
> > > 
> > >   currently, when mmapped write is done to a file backed by ext3, the
> > > filesystem does nothing to make sure blocks will be available when we need
> > > to write them out.

I thought we'd actually fixed this.  I guess we didn't.  I think what
we did do was to ensure that a subsequent fsync()/msync() would
reliably report the data loss (has anyone tested this in the past few
years??).  This is something, but it's quite lame.

> > Hmm, you've done all of this work already, so this isn't the best time
> > to suggest this, but I wonder if we've explored all of the
> > alternatives that might allow for a less drastic set of changes to
> > ext3, just out of stability's sake.
>   Yeah, I understand that and I've been also thinking for some time whether
> I cannot avoid implementing block reservation but I haven't come up with
> anything really acceptable. Moreover, unless we write via mmap to a sparse
> file, the code paths taken are changed only a little (only when and how
> we account for allocated blocks)...
> 
> > How often do legitimate workloads mmap a sparse file then write into
> > it?  As I recall, the original POSIX.1 spec didn't allow mmap beyond
> > the end of the file; this I believe was lifted later on (at least I
> > don't see it in SUSv3 spec).
>   Well, mmap beyond EOF is still undefined AFAIK (although Linux
> traditionally supports it) but mmap of sparse files was always supposed
> to work. My favorite user of sparse-file mmap is Berkeley DB, some torrent
> clients do that as well and I believe there are others. So it's not the most
> common thing but it happens often enough.

Yes, people do this.  With a 64-bit address space they create a
gargantuan mmap of the entire database and just populate teeny bits of
it simply with CPU stores.  They'd be unhappy if the kernel started
instantiating every block within the mmap()!

> > If it's not all that common, then other options are:
> > 
> > 1) Fail an mmap with EINVAL if there is an attempt to map a file
> > region which is either sparse or extends beyond the end of a file.
> > This is probably not a great alternative, but it's a possibility.
>   This is no-go IMHO. We would surely get lots of users complaining...
> 
> > 2) Allocate all of the pages that are not allocated at mmap time.
> > Since ext3 doesn't have space for an uninitialized bit, we'd have to
> > either (2a) forcing a disk write out for all of the newly initialized
> > pages, or (2b) keep track of the allocated disk blocks in memory, but
> > don't actually write the block mappings to the indirect blocks until
> > the blocks are actually written out.  (This last might be just as
> > complex, alas).
>   Doing allocation at mmap time does not really work - on each mmap we
> would have to map blocks for the whole file which would make mmap really
> expensive operation. Doing it at page-fault as you suggest in (2a) works
> (that's the second plausible option IMO) but the increased fragmentation
> and thus loss of performance is rather noticeable. I don't have current
> numbers but when I tried that last year Berkeley DB was like two or three
> times slower.

ouch.

Can we fix the layout problem?  Are reservation windows of no use here?

>   In your (2b) suggestion, I don't see how we would avoid leaking allocated
> blocks when we crash before writing allocation to indirect block. Also the
> fragmentation problem which seems to be the main source of performance
> issues would stay the same.
>   
> > 3) Keep a global counter of sparse blocks which are mapped at mmap()
> > time, and update it as blocks are allocated, or when the region is
> > freed at munmap() time.
>   Here again I see the problem that mapping all file blocks at mmap time
> is rather expensive and so does not seem viable to me. Also the
> overestimation of needed blocks could be rather huge.

When I did ext2 delayed allocation back in, err, 2001 I had
considerable trouble working out how many blocks to actually reserve
for a file block, because it also had to reserve the indirect blocks. 
One file block allocation can result in reserving four disk blocks! 
And iirc it was not possible with existing in-core data structures to
work out whether all four blocks needed reserving until the actual
block allocation had occurred.  So I ended up reserving the worst-case
number of indirects, based upon the file offset.  If the disk ran out
of "space" I'd do a forced writeback to empty all the reservations and
would then take a look to see if the disk was _really_ out of space.

Is all of this an issue with this work?  If so, what approach did you
take?

> > #3 might be much simpler, at the end of the day.  Note that there are
> > some Japanese customers that really freaked with ext4 just because it
> > was *different*, and begged a distribution not to ship ext4 because it
> > might destablize their customers.  Not that I think we are obliged to
> > listen to some of the more extremely conservative customers, but there
> > was something nice about telling people (well, if you want something
> > which is nice and stable and conservative, you can pick ext3).
>   I'm aware of this. Actually, the user observable differences should be
> rather minimal. The only one I'm aware of is that you can get SIGSEGV at
> page fault time because the filesystem runs out of disk space (or out of
> disk quota) which seems better than throwing away the data later. Also I
> don't think anybody serious runs systems close to ENOSPC regularly and if
> that happens accidentally, manual intervention is usually needed anyway...

Gee.  I remember people having issues with forcing the SEGV at
pagefault time.  It _is_ a behaviour change: the application might be
about to free up some disk space, so the msync() would have succeeded
anyway.  

iirc another issue was that the standards (posix?) don't anticipate
getting a SEGV in response to ENOSPC.  There might have been other
concerns - it's all foggy now.


Our general answer to this overall problem is: "run msync() and check
the result".  That's a bit weaselly, but it's not a _bad_ answer. 
After all, there might be an EIO as well!  So a good application should
be checking for both ENOSPC and EIO.  Your patches only address the
ENOSPC.

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