Re: [RFC] improve space utilization on off-sized raid devices

["Thomas Schmidt" <Schmidt-T@gmx.de>]

On Thursday 01 December 2011 09:55:27 Arne Jansen wrote:
> As RAID0 is already not a strict 'all disks or none', I like the idea=
 to
> have it even more dynamic to reach full optimization. But I'd like to=
 see
> some properties conserved:
>  a) In case of even size disks, the stripes should always be full siz=
e, not
>    n - 1
>  b) Minor variations in the used space per disk due to metadata chunk=
s
> should not lead to deviation from a)
>  c) The algorithms should not give weird results under unconventional
> setups. Some theoretical background would be nice=20

Resent because it did not appear on the ML for about 4h.
KMail's acting up.

Sorry to only get back to you now, I must have missed your mail somehow=
=2E

The problem is the shrinking stripe width with unmatched devices. Once =
it hits devs_min-1 it's over. My solution is to try to keep the stripe =
width constant.
The sorting then takes care of selecting the right devices.

It's simply: space / min-hight =3D max-width
a) is dictated by math
Since circumstances change (add, rm devs, rounding, ...) it is calculat=
ed again at every allocation. The result is then rounded to the nearest=
 multiple of devs_increment. This takes care of b).

The code may look wiered but should be identical to the mathematical
floor(Space / min-hight + increment/2) if considered together with the =
round down already present in the line after my patch.

The two ifs should safeguard against weird stuff by limiting the result=
 to sane values.

I include an updated patch below. It's again written for and tested wit=
h 3.0.0 but diff3 worked nicely for applying it to 3.3-rc1.

--- volumes.c.orig      2012-01-20 16:59:31.000000000 +0100
+++ volumes.c   2012-01-24 11:24:07.261401805 +0100
@@ -2329,6 +2329,8 @@
        u64 stripe_size;
        u64 num_bytes;
        int ndevs;
+       u64 fs_total_avail;
+       int opt_ndevs;
        int i;
        int j;
=20
@@ -2448,6 +2450,7 @@
                devices_info[ndevs].total_avail =3D total_avail;
                devices_info[ndevs].dev =3D device;
                ++ndevs;
+               fs_total_avail +=3D total_avail;
        }
=20
        /*
@@ -2456,6 +2459,16 @@
        sort(devices_info, ndevs, sizeof(struct btrfs_device_info),
             btrfs_cmp_device_info, NULL);
=20
+       /*
+       * do not allocate space on all devices
+       * instead balance free space to maximise space utilization
+       */
+       opt_ndevs =3D (fs_total_avail*2 + devs_increment*devices_info[0=
].total_avail) / (devices_info[0].total_avail*2);
+       if (opt_ndevs < devs_min)
+               opt_ndevs =3D devs_min;
+       if (ndevs > opt_ndevs)
+               ndevs =3D opt_ndevs;
+
        /* round down to number of usable stripes */
        ndevs -=3D ndevs % devs_increment;

--=20
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