Re: [PATCH] ceph: make nearfull sync writes opt-in

LGTM.

Reviewed-by: Xiubo Li <[email protected]>

Alex Markuze <[email protected]> 于2026年7月6日周一 07:03写道：
>
> The kernel CephFS client has historically treated a cluster or pool
> NEARFULL condition as a request to force successful writes through
> generic_write_sync().  That effectively turns otherwise buffered writes
> into synchronous writes and can cause a severe throughput drop as soon
> as a single OSD or the file data pool crosses the nearfull threshold.
>
> On modern large clusters, NEARFULL is primarily an operator health
> signal rather than an immediate client-side capacity failure.  Operators
> can still have substantial usable capacity while a cluster is
> rebalancing, splitting PGs, or expanding onto new devices.  RBD, RGW and
> the userspace CephFS client do not impose this extra client-side
> sync-write throttle, so the kernel client behavior is surprising and
> operationally painful.
>
> Change the default behavior so NEARFULL no longer changes normal
> write-sync semantics.  FULL and pool FULL still fail with -ENOSPC, and
> explicitly synchronous writes continue to be synced by
> generic_write_sync().
>
> Add a nearfull_sync mount option for deployments that want the legacy
> backpressure behavior.  When this option is set, successful writes are
> promoted to IOCB_DSYNC if the cluster or file data pool is marked
> NEARFULL, preserving the old behavior for conservative deployments.
>
> Link: https://tracker.ceph.com/issues/74849
> Signed-off-by: Alex Markuze <[email protected]>
> ---
>  Documentation/filesystems/ceph.rst |  6 ++++++
>  fs/ceph/file.c                     |  8 +++++---
>  fs/ceph/super.c                    | 10 ++++++++++
>  fs/ceph/super.h                    |  1 +
>  4 files changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/filesystems/ceph.rst b/Documentation/filesystems/ceph.rst
> index 6d2276a87a5a..ee2ca0c0c654 100644
> --- a/Documentation/filesystems/ceph.rst
> +++ b/Documentation/filesystems/ceph.rst
> @@ -194,6 +194,12 @@ Mount Options
>          copies.  Currently, it's only used in copy_file_range, which will revert
>          to the default VFS implementation if this option is used.
>
> +  nearfull_sync
> +       Force written data to stable storage when the cluster or file data pool is
> +       marked NEARFULL. This restores the legacy client-side backpressure
> +       behavior. By default, CephFS writes are not forced synchronous solely
> +       because of NEARFULL.
> +
>    recover_session=<no|clean>
>         Set auto reconnect mode in the case where the client is blocklisted. The
>         available modes are "no" and "clean". The default is "no".
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index 3823671ab95a..0ae9f09aa233 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -2400,7 +2400,8 @@ static ssize_t ceph_splice_read(struct file *in, loff_t *ppos,
>   * dropping our cap refs and allowing the pending snap to logically
>   * complete _before_ this write occurs.
>   *
> - * If we are near ENOSPC, write synchronously.
> + * If requested, nearfull writes are synced to preserve the legacy
> + * client-side backpressure behavior.
>   */
>  static ssize_t ceph_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  {
> @@ -2568,8 +2569,9 @@ static ssize_t ceph_write_iter(struct kiocb *iocb, struct iov_iter *from)
>         }
>
>         if (written >= 0) {
> -               if ((map_flags & CEPH_OSDMAP_NEARFULL) ||
> -                   (pool_flags & CEPH_POOL_FLAG_NEARFULL))
> +               if (ceph_test_mount_opt(fsc, NEARFULL_SYNC) &&
> +                   ((map_flags & CEPH_OSDMAP_NEARFULL) ||
> +                    (pool_flags & CEPH_POOL_FLAG_NEARFULL)))
>                         iocb->ki_flags |= IOCB_DSYNC;
>                 written = generic_write_sync(iocb, written);
>         }
> diff --git a/fs/ceph/super.c b/fs/ceph/super.c
> index 0bbd38933f0e..4d129483e345 100644
> --- a/fs/ceph/super.c
> +++ b/fs/ceph/super.c
> @@ -178,6 +178,7 @@ enum {
>         Opt_pagecache,
>         Opt_sparseread,
>         Opt_lazyio,
> +       Opt_nearfull_sync,
>  };
>
>  enum ceph_recover_session_mode {
> @@ -207,6 +208,7 @@ static const struct fs_parameter_spec ceph_mount_parameters[] = {
>         fsparam_flag_no ("lazyio",                      Opt_lazyio),
>         fsparam_string  ("mds_namespace",               Opt_mds_namespace),
>         fsparam_string  ("mon_addr",                    Opt_mon_addr),
> +       fsparam_flag_no ("nearfull_sync",               Opt_nearfull_sync),
>         fsparam_flag_no ("poolperm",                    Opt_poolperm),
>         fsparam_flag_no ("quotadf",                     Opt_quotadf),
>         fsparam_u32     ("rasize",                      Opt_rasize),
> @@ -601,6 +603,12 @@ static int ceph_parse_mount_param(struct fs_context *fc,
>                 else
>                         fsopt->flags |= CEPH_MOUNT_OPT_LAZYIO;
>                 break;
> +       case Opt_nearfull_sync:
> +               if (result.negated)
> +                       fsopt->flags &= ~CEPH_MOUNT_OPT_NEARFULL_SYNC;
> +               else
> +                       fsopt->flags |= CEPH_MOUNT_OPT_NEARFULL_SYNC;
> +               break;
>         case Opt_test_dummy_encryption:
>  #ifdef CONFIG_FS_ENCRYPTION
>                 fscrypt_free_dummy_policy(&fsopt->dummy_enc_policy);
> @@ -759,6 +767,8 @@ static int ceph_show_options(struct seq_file *m, struct dentry *root)
>                 seq_puts(m, ",sparseread");
>         if (fsopt->flags & CEPH_MOUNT_OPT_LAZYIO)
>                 seq_puts(m, ",lazyio");
> +       if (fsopt->flags & CEPH_MOUNT_OPT_NEARFULL_SYNC)
> +               seq_puts(m, ",nearfull_sync");
>
>         fscrypt_show_test_dummy_encryption(m, ',', root->d_sb);
>
> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> index 3737ea7ed88b..632ffb1b401e 100644
> --- a/fs/ceph/super.h
> +++ b/fs/ceph/super.h
> @@ -46,6 +46,7 @@
>  #define CEPH_MOUNT_OPT_NOPAGECACHE     (1<<16) /* bypass pagecache altogether */
>  #define CEPH_MOUNT_OPT_SPARSEREAD      (1<<17) /* always do sparse reads */
>  #define CEPH_MOUNT_OPT_LAZYIO          (1<<18) /* force lazyio for all file opens */
> +#define CEPH_MOUNT_OPT_NEARFULL_SYNC   (1<<19) /* sync writes when nearfull */
>
>  #define CEPH_MOUNT_OPT_DEFAULT                 \
>         (CEPH_MOUNT_OPT_DCACHE |                \
> --
> 2.34.1
>
>