New tunable parameters

[imp]

While the new tunables are not intended for day-to-day tweak, it is the question of parity with Illumos - if the need to tinker with these variable ever arise the Linux way of doing so is module parameter.

Commit 55d85d5 (backport of the upstream changes)
replaced three hardcoded constants:

#define SYNC_PASS_DEFERRED_FREE 2 /* defer frees after this pass */
#define SYNC_PASS_DONT_COMPRESS 4 /* don't compress after this pass */
#define SYNC_PASS_REWRITE       1 /* rewrite new bps after this pass */

with a tunable parameters:

int zfs_sync_pass_deferred_free = 2; /* defer frees starting in this pass */
int zfs_sync_pass_dont_compress = 5; /* don't compress starting in this pass */
int zfs_sync_pass_rewrite = 2;       /* rewrite new bps starting in this pass */

This commit makes these tunables available as module parameters in Linux.

[behlendorf]

@imp I thought about this when originally porting the patch but opted against it at the time because changing these values can lead to pathological performance issues. This isn't the sort of thing anyone but a ZFS developer should be tinkering with. On Illumos the story is a little different because the change these values you would need to use mdb, but on Linux it's a lot easier. That said, we already expose some other knobs which are dangerous so perhaps this is OK. Have you been investigating changing these values?

[imp]

@behlendorf I trip over this while investigating something else at zio.c. I initially wrote this patch with intention to play with these tunables, but than the need disappeared. The patch sat in my queue for some time until a periodic git repo clean up. At that point I decided it is better to offer it upstream than remove it.
I totally agree with you that these variables are not for casual consumption. However, if the need to tweak ever arise - it would be a shame to have to recompile the code.
I leave it for you to decide whether to merge it or not. If not I'll leave this branch here just in case.