Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate UAPI

Ric Wheeler
2012-12-07T21:49:04Z

On 12/07/2012 04:43 PM, Chris Mason wrote:
> On Fri, Dec 07, 2012 at 02:27:43PM -0700, Theodore Ts'o wrote:
>> On Fri, Dec 07, 2012 at 04:09:32PM -0500, Chris Mason wrote:
>>> Persistent trim is what I had in mind, but there are other ideas that do
>>> imply a change in behavior as well. Can we safely assume this feature
>>> won't matter on spinning media? New features like persistent
>>> trim do make it much easier to solve securely, and using a bit for it
>>> means we can toss back an error to the app if the underlying storage
>>> isn't safe.
>> We originally implemented no hide stale for spinning media. Some
>> folks have claimed that for XFS their superior technology means that
>> no hide stale doesn't buy them anything for HDD's. I'm not entirely
>> sure I buy this, since if you need to update metadata, it means at
>> least one extra seek for each random write into 4k preallocated space,
>> and 7200 RPM disks only have about 200 seeks per second.
> True, 7200 RPM disks are slow, but even allowing them to expose stale
> data just makes them a little less slow.
>
> I know it's against the rules to pretend that disks don't matter. But
> really, once you're doing random IO into a spindle you've given up on
> performance anyway.
>
> -chris

That's right.

And equally true, once you have moved the disk heads to that track, you can
write a lot as cheaply as a little (i.e., do 1MB instead of 4KB). That will also
avoid fragmentation of the extents.

I think it would be good to see how much that gets back for us,

Ric