Re: [RFC][PATCH 0/3] add FALLOC_FL_NO_HIDE_STALE flag in fallocate

[Eric Sandeen <sandeen@redhat.com>]

On 4/17/12 1:43 PM, Ted Ts'o wrote:
> On Tue, Apr 17, 2012 at 01:59:37PM -0400, Ric Wheeler wrote:
>>
>> You could get both security and avoid the run time hit by fully
>> writing the file or by having a variation that relied on "discard"
>> (i.e., no need to zero data if we can discard or track it as
>> unwritten).
> 
> It's certainly the case that if the device supports persistent
> discard, something which we definitely *should* do is to send the
> discard at fallocate time and then mark the space as initialized.
> 
> Unfortunately, not all devices, and in particular no HDD's for which I
> aware support persistent discard.  And, writing all zero's to the file
> is in fact what a number of programs for which I am aware (including
> an enterprise database) are doing, precisely because they tend to
> write into the fallocated space in a somewhat random order, and the
> extent conversion costs is in fact quite significant.  But writing all
> zero's to the file before you can use it is quite costly; at the very
> least it burns disk bandwidth --- one of the main motivations of
> fallocate was to avoid needing to do a "write all zero pass", and
> while it does solve the problem for some use cases (such as DVR's),
> it's not a complete solution.

Can we please start with profiling the workload causing trouble, see why
ext4 takes such a hit, and see if anything can be done there to fix
it surgically, rather than just throwing this big hammer at it?

In my (admittedly quick, hacky) test, xfs suffed about a 1% perf degradation,
ext4 about 8%.  Until we at least know why ext4 is so much worse, I'll
signal a strong NAK for this change, for whatever may or may not be worth.  :)

-Eric