On Mon, 2007-05-07 at 16:31 -0700, Andrew Morton wrote: > 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. > In this case, since the number of blocks to preallocate (eg. N=10GB) is clear, we could improve the current reservation code, to allow callers explicitly ask for a new window that have the minimum N free blocks for the blocks-to-preallocated(rather than just have at least 1 free blocks). Before the ext4_fallocate() is called, the right reservation window size is set with the flag to indicating "please spend time if needed to find a window covers at least N free blocks". So for ex4 block mapped files, later when glibc is doing allocation and zeroing, the ext4 block-mapped allocator will knows to reserve the right amount of free blocks before allocating and zeroing 10GB space. I am not sure whether this worth the effort though. > 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
