Actually it seems to be 3/4:
dsl_pool.c
391 zfs_write_limit_max = ptob(physmem) >>
zfs_write_limit_shift;
392 zfs_write_limit_inflated = MAX(zfs_write_limit_min,
393 spa_get_asize(dp->dp_spa, zfs_write_limit_max));
While spa_get_asize is:
spa_misc.c
1249 uint64_t
1250 spa_get_asize(spa_t *spa, uint64_t lsize)
1251 {
1252 /*
1253 * For now, the worst case is 512-byte RAID-Z blocks, in which
1254 * case the space requirement is exactly 2x; so just assume
that.
1255 * Add to this the fact that we can have up to 3 DVAs per bp,
and
1256 * we have to multiply by a total of 6x.
1257 */
1258 return (lsize * 6);
1259 }
Which will result in:
zfs_write_limit_inflated = MAX((32 << 20), (ptob(physmem) >> 3) * 6);
Bob Friesenhahn wrote:
> Even if I set zfs_write_limit_override to 8053063680 I am unable to
> achieve the massive writes that Solaris 10 (141415-03) sends to my
> drive array by default.
>
> When I read the blog entry at
> http://blogs.sun.com/roch/entry/the_new_zfs_write_throttle, I see this
> statement:
>
> "The new code keeps track of the amount of data accepted in a TXG and
> the time it takes to sync. It dynamically adjusts that amount so that
> each TXG sync takes about 5 seconds (txg_time variable). It also
> clamps the limit to no more than 1/8th of physical memory."
>
> On my system I see that the "about 5 seconds" rule is being followed,
> but see no sign of clamping the limit to no more than 1/8th of
> physical memory. There is no sign of clamping at all. The writen
> data is captured and does take about 5 seconds to write (good
> estimate).
>
> On my system with 20GB of RAM, and ARC memory limit set to 10GB
> (zfs:zfs_arc_max = 0x280000000), the maximum zfs_write_limit_override
> value I can set is on the order of 8053063680, yet this results in a
> much smaller amount of data being written per write cycle than the
> Solaris 10 default operation. The default operation is 24 seconds of
> no write activity followed by 5 seconds of write.
>
> On my system, 1/8 of memory would be 2.5GB. If I set the
> zfs_write_limit_override value to 2684354560 then it seems that about
> 1.2 seconds of data is captured for write. In this case I see 5
> seconds of no write followed by maybe a second of write.
>
> This causes me to believe that the algorithm is not implemented as
> described in Solaris 10.
>
> Bob
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