On Wed, Jul 1, 2020 at 8:57 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> Container folks are complaining that dnf/yum issues too many sync while
> installing packages and this slows down the image build. Build
> requirement is such that they don't care if a node goes down while
> build was still going on. In that case, they will simply throw away
> unfinished layer and start new build. So they don't care about syncing
> intermediate state to the disk and hence don't want to pay the price
> associated with sync.
>
> So they are asking for mount options where they can disable sync on overlay
> mount point.
>
> They primarily seem to have two use cases.
>
> - For building images, they will mount overlay with nosync and then sync
> upper layer after unmounting overlay and reuse upper as lower for next
> layer.
>
> - For running containers, they don't seem to care about syncing upper
> layer because if node goes down, they will simply throw away upper
> layer and create a fresh one.
>
> So this patch provides two mount options "sync=off" and "sync=fs".
> First option disables all forms of sync. Now it is caller's responsibility
> to throw away upper if system crashes or shuts down and start fresh.
>
> Opption "sync=fs" disables all forms of sync except syncfs/umount/remount.
typo: Opption
> This is basically useful for image build where we want to persist upper
> layer only after operation is complete and upper will be renamed and
> reused as lower for next layer build.
>
> With sync=off, I am seeing roughly 20% speed up in my VM where I am just
> installing emacs in an image. Installation time drops from 31 seconds to
> 25 seconds when nosync option is used. This is for the case of building on top
> of an image where all packages are already cached. That way I take
> out the network operations latency out of the measurement.
>
> With sync=fs, I am seeing roughly 12% speed up.
>
> Giuseppe is also looking to cut down on number of iops done on the
> disk. He is complaining that often in cloud their VMs are throttled
> if they cross the limit. This option can help them where they reduce
> number of iops (by cutting down on frequent sync and writebacks).
>
> Signed-off-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> ---
> Documentation/filesystems/overlayfs.rst | 16 +++++++++++
> fs/overlayfs/copy_up.c | 12 +++++---
> fs/overlayfs/file.c | 13 ++++++++-
> fs/overlayfs/ovl_entry.h | 2 ++
> fs/overlayfs/readdir.c | 4 +++
> fs/overlayfs/super.c | 37 +++++++++++++++++++++++--
> 6 files changed, 76 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index 660dbaf0b9b8..4e8e44ee31aa 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -563,6 +563,22 @@ This verification may cause significant overhead in some cases.
> Note: the mount options index=off,nfs_export=on are conflicting and will
> result in an error.
>
> +Disable sync
> +------------
> +By default, overlay skips sync on files residing on a lower layer. It
> +is possible to skip sync operations for files on the upper layer as well
> +with the "sync=off" and "sync=fs" mount option.
> +
> +"sync=off" option disables all forms of sync from overlay, including the
> +one done at umount/remount. If system crashes or shuts down data, user
> +should throw away upper and start fresh.
> +
> +"sync=fs" option disables all forms of sync except full filesystem
> +sync which is done at syncfs/remount/mount time. This is useful for
> +use cases like container image build which want upper to persist
> +only if operation has finished. If system crashes before image
> +layer formation is complete, tools will discard upper and start
> +fresh.
>
Nice.
> Testsuite
> ---------
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 79dd052c7dbf..e576929d619d 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -128,7 +128,8 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
> return error;
> }
>
> -static int ovl_copy_up_data(struct path *old, struct path *new, loff_t len)
> +static int ovl_copy_up_data(struct ovl_fs *ofs, struct path *old,
> + struct path *new, loff_t len)
> {
> struct file *old_file;
> struct file *new_file;
> @@ -218,7 +219,7 @@ static int ovl_copy_up_data(struct path *old, struct path *new, loff_t len)
> len -= bytes;
> }
> out:
> - if (!error)
> + if (!error && (!ofs->config.nosync && !ofs->config.syncfs))
> error = vfs_fsync(new_file, 0);
> fput(new_file);
> out_fput:
> @@ -484,6 +485,7 @@ static int ovl_link_up(struct ovl_copy_up_ctx *c)
>
> static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
> {
> + struct ovl_fs *ofs = OVL_FS(c->dentry->d_sb);
> int err;
>
> /*
> @@ -499,7 +501,8 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
> upperpath.dentry = temp;
>
> ovl_path_lowerdata(c->dentry, &datapath);
> - err = ovl_copy_up_data(&datapath, &upperpath, c->stat.size);
> + err = ovl_copy_up_data(ofs, &datapath, &upperpath,
> + c->stat.size);
> if (err)
> return err;
> }
> @@ -784,6 +787,7 @@ static bool ovl_need_meta_copy_up(struct dentry *dentry, umode_t mode,
> /* Copy up data of an inode which was copied up metadata only in the past. */
> static int ovl_copy_up_meta_inode_data(struct ovl_copy_up_ctx *c)
> {
> + struct ovl_fs *ofs = OVL_FS(c->dentry->d_sb);
> struct path upperpath, datapath;
> int err;
> char *capability = NULL;
> @@ -804,7 +808,7 @@ static int ovl_copy_up_meta_inode_data(struct ovl_copy_up_ctx *c)
> goto out;
> }
>
> - err = ovl_copy_up_data(&datapath, &upperpath, c->stat.size);
> + err = ovl_copy_up_data(ofs, &datapath, &upperpath, c->stat.size);
> if (err)
> goto out_free;
>
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 01820e654a21..2a01fb8ba897 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -329,6 +329,8 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> struct fd real;
> const struct cred *old_cred;
> ssize_t ret;
> + int ifl = iocb->ki_flags;
> + struct ovl_fs *ofs;
>
> if (!iov_iter_count(iter))
> return 0;
> @@ -344,11 +346,15 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> if (ret)
> goto out_unlock;
>
> + ofs = OVL_FS(inode->i_sb);
> + if (ofs->config.nosync || ofs->config.syncfs)
> + ifl &= ~(IOCB_DSYNC | IOCB_SYNC);
> +
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> if (is_sync_kiocb(iocb)) {
> file_start_write(real.file);
> ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
> - ovl_iocb_to_rwf(iocb->ki_flags));
> + ovl_iocb_to_rwf(ifl));
> file_end_write(real.file);
> /* Update size */
> ovl_copyattr(ovl_inode_real(inode), inode);
> @@ -368,6 +374,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> real.flags = 0;
> aio_req->orig_iocb = iocb;
> kiocb_clone(&aio_req->iocb, iocb, real.file);
> + aio_req->iocb.ki_flags = ifl;
> aio_req->iocb.ki_complete = ovl_aio_rw_complete;
> ret = vfs_iocb_iter_write(real.file, &aio_req->iocb, iter);
> if (ret != -EIOCBQUEUED)
> @@ -430,6 +437,10 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> struct fd real;
> const struct cred *old_cred;
> int ret;
> + struct ovl_fs *ofs = OVL_FS(file_inode(file)->i_sb);
> +
> + if (ofs->config.nosync || ofs->config.syncfs)
> + return 0;
>
> ret = ovl_real_fdget_meta(file, &real, !datasync);
> if (ret)
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index b429c80879ee..473508f1a1da 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -17,6 +17,8 @@ struct ovl_config {
> bool nfs_export;
> int xino;
> bool metacopy;
> + bool nosync;
> + bool syncfs;
> };
>
> struct ovl_sb {
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 6918b98faeb6..970319ca1623 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -858,11 +858,15 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
> struct ovl_dir_file *od = file->private_data;
> struct dentry *dentry = file->f_path.dentry;
> struct file *realfile = od->realfile;
> + struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
>
> /* Nothing to sync for lower */
> if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
> return 0;
>
> + if (ofs->config.nosync || ofs->config.syncfs)
> + return 0;
> +
Generally looks good, but those test conditions are quite weird IMO.
I would go for either enum or flags, but not up to me to decide.
But at the very least use inline helpers ovl_should_fsync(),
ovl_should_syncfs(), because if we want to add new sync modes or
whatever going over all those conditions and
changing them would be sub-optimal.
Thanks,
Amir.
