diff options
| -rw-r--r-- | man/man5/zfs-module-parameters.5 | 16 | ||||
| -rw-r--r-- | module/zfs/zio.c | 18 |
2 files changed, 30 insertions, 4 deletions
diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5 index 3ed7bc6e4..7fb24d275 100644 --- a/man/man5/zfs-module-parameters.5 +++ b/man/man5/zfs-module-parameters.5 @@ -2478,9 +2478,21 @@ Default value: \fB25\fR. \fBzfs_sync_pass_dont_compress\fR (int) .ad .RS 12n -Don't compress starting in this pass +Starting in this sync pass, we disable compression (including of metadata). +With the default setting, in practice, we don't have this many sync passes, +so this has no effect. +.sp +The original intent was that disabling compression would help the sync passes +to converge. However, in practice disabling compression increases the average +number of sync passes, because when we turn compression off, a lot of block's +size will change and thus we have to re-allocate (not overwrite) them. It +also increases the number of 128KB allocations (e.g. for indirect blocks and +spacemaps) because these will not be compressed. The 128K allocations are +especially detrimental to performance on highly fragmented systems, which may +have very few free segments of this size, and may need to load new metaslabs +to satisfy 128K allocations. .sp -Default value: \fB5\fR. +Default value: \fB8\fR. .RE .sp diff --git a/module/zfs/zio.c b/module/zfs/zio.c index 80a2dbc82..a6bf8a27b 100644 --- a/module/zfs/zio.c +++ b/module/zfs/zio.c @@ -20,7 +20,7 @@ */ /* * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved. - * Copyright (c) 2011, 2018 by Delphix. All rights reserved. + * Copyright (c) 2011, 2019 by Delphix. All rights reserved. * Copyright (c) 2011 Nexenta Systems, Inc. All rights reserved. * Copyright (c) 2017, Intel Corporation. */ @@ -96,9 +96,23 @@ int zio_slow_io_ms = (30 * MILLISEC); * * The 'zfs_sync_pass_deferred_free' pass must be greater than 1 to ensure that * regular blocks are not deferred. + * + * Starting in sync pass 8 (zfs_sync_pass_dont_compress), we disable + * compression (including of metadata). In practice, we don't have this + * many sync passes, so this has no effect. + * + * The original intent was that disabling compression would help the sync + * passes to converge. However, in practice disabling compression increases + * the average number of sync passes, because when we turn compression off, a + * lot of block's size will change and thus we have to re-allocate (not + * overwrite) them. It also increases the number of 128KB allocations (e.g. + * for indirect blocks and spacemaps) because these will not be compressed. + * The 128K allocations are especially detrimental to performance on highly + * fragmented systems, which may have very few free segments of this size, + * and may need to load new metaslabs to satisfy 128K allocations. */ 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_dont_compress = 8; /* don't compress starting in this pass */ int zfs_sync_pass_rewrite = 2; /* rewrite new bps starting in this pass */ /* |