HEADS UP - HAMMER work

[Dennis Melentyev]

Hi!

2008/11/14 Oliver Fromme <check+kabwj800rsns8lml at fromme.com>:
> Matthew Dillon wrote:
>  >    64-bit directory hash encoding (for smaller filenames out of bounds
>  >    indices just store a 0).
>  >
>  >    aaaaa       name[0] & 0x1F
>  >    bbbbb       name[1] & 0x1F
>  >    ccccc       name[2] & 0x1F
>  >    mmmmmm      crc32(name + 3, len - 5) and some xor magic -> 6 bits
>  >    yyyyy       name[len-2] & 0x1F
>  >    zzzzz       name[len-1] & 0x1F
>  >    h[31]       crc32(name, len) (entire filename)
>  >
>  >    0aaaaabbbbbccccc mmmmmmyyyyyzzzzz hhhhhhhhhhhhhhhh hhhhhhhhhhhhhhh0
>  > [...]
. ....

>
> You already mentioned it.  That's exactly the problem
> that I'm seeing ...  I'm not sure whether a[], b[], c[],
> y[] and z[] buy you anything in practice.
>
> If a single directory contains a huge number of files,
> it is likely they are all of the same type, e.g. it could
> be a collection of images or whatever.  That means they
> all have the same extension (e.g. .jpg), so y[] and z[]
> are useless.
>
> Furthermore, it isn't completely unlikely that they even
> begin with the same prefix.  For example, all of my
> digital camera pics are named "img%05d.jpg".  Admittedly
> those aren't millions (but more than 10k anyway), and
> I'm not stupid enough to collect them in a single
> directory.  ;-)
>
> Another example:  The cache directory of my Opera browser.
> It contains several thousands of files all beginning
> with "opr*".
>
> It might be a good idea to make a small survey, i.e. find
> people who actually _do_ have directories with a huge
> number of files in them (and I mean more than just a few
> thousands), and ask them what the filenames typically look
> like.
>
> An obvious improvement would be to store name[d-2] and
> name[d-1] in y[] and z[], respectively, where d is the
> location of the last dot in the filename, if any, or the
> location of the terminating zero if there is no dot.
> In other words:  Ignore the extension when identifying
> y[] and z[].  Finding the last dot shouldn't be more
> computationally expensive than strlen(name), so this
> shouldn't be a problem.

I do agree with Oliver. But have another proposal:
Also, I doubt that there are usually more than 1-2 affected
directories per host. And usually, file names has very similar
pattern.

Sysctl/some-other-tunable with some kind of mask would be great for
fine-tuning (and just useless for the 90% of users).

like:
sysctl.hammer.dirhash.hashmask.prefix=1 (Starting at first filename
byte, 3 bytes fixed length)
sysctl.hammer.dirhash.hashmask.suffix=-1 (Starting last byte, 2 bytes length)

This way, admins would be able to re-tune it to their particular needs.

Just my 0.02UAH
-- 
Dennis Melentyev