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Default to zfs_dmu_offset_next_sync=1

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Strict hole reporting was previously disabled by default as a
performance optimization.  However, this has lead to confusion
over the expected behavior and a variety of workarounds being
adopted by consumers of ZFS.  Change the default behavior to
always report holes and force the TXG sync.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12746
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Brian Behlendorf 2021-11-30 10:38:09 -08:00 committed by GitHub
parent 5dc6fc2b73
commit 05b3eb6d23
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2 changed files with 12 additions and 8 deletions
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8
man/man4/zfs.4
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@ -1591,12 +1591,12 @@ Allow no-operation writes.
The occurrence of nopwrites will further depend on other pool properties
.Pq i.a. the checksumming and compression algorithms .
.
.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 0 Ns | Ns 1 Pq int
.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 1 Ns | Ns 0 Pq int
Enable forcing TXG sync to find holes.
When enabled forces ZFS to act like prior versions when
When enabled forces ZFS to sync data when
.Sy SEEK_HOLE No or Sy SEEK_DATA
flags are used, which, when a dnode is dirty,
causes TXGs to be synced so that this data can be found.
flags are used allowing holes in a file to be accurately reported.
When disabled holes will not be reported in recently dirtied files.
.
.It Sy zfs_pd_bytes_max Ns = Ns Sy 52428800 Ns B Po 50MB Pc Pq int
The number of bytes which should be prefetched during a pool traversal, like

12
module/zfs/dmu.c
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@ -73,9 +73,13 @@ int zfs_nopwrite_enabled = 1;
unsigned long zfs_per_txg_dirty_frees_percent = 5;
/*
* Enable/disable forcing txg sync when dirty in dmu_offset_next.
* Enable/disable forcing txg sync when dirty checking for holes with lseek().
* By default this is enabled to ensure accurate hole reporting, it can result
* in a significant performance penalty for lseek(SEEK_HOLE) heavy workloads.
* Disabling this option will result in holes never being reported in dirty
* files which is always safe.
*/
int zfs_dmu_offset_next_sync = 0;
int zfs_dmu_offset_next_sync = 1;
/*
* Limit the amount we can prefetch with one call to this amount. This
@ -2107,8 +2111,8 @@ restart:
* If the zfs_dmu_offset_next_sync module option is enabled
* then strict hole reporting has been requested. Dirty
* dnodes must be synced to disk to accurately report all
* holes. When disabled (the default) dirty dnodes are
* reported to not have any holes which is always safe.
* holes. When disabled dirty dnodes are reported to not
* have any holes which is always safe.
*
* When called by zfs_holey_common() the zp->z_rangelock
* is held to prevent zfs_write() and mmap writeback from