Re: VFS hot tracking: How to calculate data temperature?

[Steven Whitehouse <swhiteho@redhat.com>]

Hi,

On Mon, 2012-11-05 at 19:46 +0800, Zhi Yong Wu wrote:
> On Mon, Nov 5, 2012 at 6:33 PM, Steven Whitehouse <swhiteho@redhat.com> wrote:
> > Hi,
> >
> > On Mon, 2012-11-05 at 16:44 +0800, Zhi Yong Wu wrote:
> >> On Mon, Nov 5, 2012 at 4:28 PM, Dave Chinner <david@fromorbit.com> wrote:
> >> > On Mon, Nov 05, 2012 at 10:35:50AM +0800, Zhi Yong Wu wrote:
> >> >> On Sat, Nov 3, 2012 at 5:27 AM, Mingming.cao <cmm@us.ibm.com> wrote:
> >> >> > On Fri, 2012-11-02 at 14:38 +0800, Zhi Yong Wu wrote:
> >> >> >> Here also has another question.
> >> >> >>
> >> >> >> How to save the file temperature among the umount to be able to
> >> >> >> preserve the file tempreture after reboot?
> >> >> >>
> >> >> >> This above is the requirement from DB product.
> >> >> >> I thought that we can save file temperature in its inode struct, that
> >> >> >> is, add one new field in struct inode, then this info will be written
> >> >> >> to disk with inode.
> >> >> >>
> >> >> >> Any comments or ideas are appreciated, thanks.
> >> >> >>
> >> >> >>
> >> >> >
> >> >> > Maybe could save the last file temperature with extended attributes.
> >> >> It seems that only ext4 has the concept of extended attributes.
> >> >
> >> > All major filesystems have xattr support. They are used extensively
> >> > by the security and integrity subsystems, for example.
> >> got it, thanks.
> >> >
> >> > Saving the information might be something that is useful to certian
> >> > applications, but lets have the people that need that functionality
> >> > spell out their requirements before discussing how or what to
> >> > implement.  Indeed, discussion shoul dreally focus on getting the
> >> > core, in-memory infrastructure sorted out first before trying to
> >> > expand the functionality further...
> >> ah, but the latest patchset need some love from experienced FS guys:).......
> >
> > There is one other possible issue with saving the data into the
> > filesystem, which is that it may disturb what you are trying to measure.
> > Some filesystems (GFS2 is one) store data for small inodes in the same
> > block as the inode itself. So that means the accesses to the saved hot
> > tracking info may potentially affect the data access times too. Also
> > there is a very limited amount of space to expand the number of fields
> > in the inode, so xattr may be the only solution, depending on how much
> > data needs to be stored in each case.
> Very good analysis, two possible issues are very meanful, thanks.
> >
> > In the GFS2 case (I don't think it is unique in this) xattrs are stored
> > out of line and having to access them in every open means an extra block
> > read per inode, which again has performance implications.
> >
> > So that is not an insurmountable problem, but something to take into
> > account in selecting a solution,
> In summary, you look like preferring to xattr as its solution.
> 

Well, that depends on exactly how large the data to be stored is, and
other factors. It will add overhead to the storage/retrieval but at
least it is fairly generic (wrt on-disk format) so likely to be easier
to retrofit to existing filesystems.

I suspect this may be one of those cases where there is no obvious right
answer and it is a case of selecting the least worst option, if that
makes sense?

Steve.