On Mon, 7 May 2007 19:14:42 -0400 Theodore Tso <tytso@xxxxxxx> wrote: > On Mon, May 07, 2007 at 03:38:56PM -0700, Andrew Morton wrote: > > > Actually, this is a non-issue. The reason that it is handled for extent-only > > > is that this is the only way to allocate space in the filesystem without > > > doing the explicit zeroing. For other filesystems (including ext3 and > > > ext4 with block-mapped files) the filesystem should return an error (e.g. > > > -EOPNOTSUPP) and glibc will do manual zero-filling of the file in userspace. > > > > It can be a bit suboptimal from the layout POV. The reservations code will > > largely save us here, but kernel support might make it a bit better. > > Actually, the reservations code won't matter, since glibc will fall > back to its current behavior, which is it will do the preallocation by > explicitly writing zeros to the file. No! Reservations code is *critical* here. Without reservations, we get disastrously-bad layout if two processes were running a large fallocate() at the same time. (This is an SMP-only problem, btw: on UP the timeslice lengths save us). My point is that even though reservations save us, we could do even-better in-kernel. But then, a smart application would bypass the glibc() fallocate() implementation and would tune the reservation window size and would use direct-IO or sync_file_range()+fadvise(FADV_DONTNEED). > This wlil result in the same > layout as if we had done the persistent preallocation, but of course > it will mean the posix_fallocate() could potentially take a long time > if you're a PVR and you're reserving a gig or two for a two hour movie > at high quality. That seems suboptimal, granted, and ideally the > application should be warned about this before it calls > posix_fallocate(). On the other hand, it's what happens today, all > the time, so applications won't be too badly surprised. A PVR implementor would take all this over and would do it themselves, for sure. > If we think applications programmers badly need to know in advance if > posix_fallocate() will be fast or slow, probably the right thing is to > define a new fpathconf() configuration option so they can query to see > whether a particular file will support a fast posix_fallocate(). I'm > not 100% convinced such complexity is really needed, but I'm willing > to be convinced.... what do folks think? > An application could do sys_fallocate(one-byte) to work out whether it's supported in-kernel, I guess. - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
