Thanks for the review and suggestions! I'll send a v2 later, two replies
inline.
On 11/5/19 10:14 AM, Aaron Lauterer wrote:
Nicely written.
I have some suggestions inline:
* splitting long sentences
* adding more info as to what is valid for the size in
special_small_blocks (taken from the zfs man page)
* rewrote the last paragraph a bit
On 10/22/19 12:33 PM, Fabian Ebner wrote:
> Signed-off-by: Fabian Ebner <f.eb...@proxmox.com>
> ---
> local-zfs.adoc | 44 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 44 insertions(+)
>
> diff --git a/local-zfs.adoc b/local-zfs.adoc
> index b4fb7db..378cbee 100644
> --- a/local-zfs.adoc
> +++ b/local-zfs.adoc
> @@ -431,3 +431,47 @@ See the `encryptionroot`, `encryption`,
`keylocation`, `keyformat` and
> `keystatus` properties, the `zfs load-key`, `zfs unload-key` and `zfs
> change-key` commands and the `Encryption` section from `man zfs`
for more
> details and advanced usage.
> +
> +
> +ZFS Special Device
> +~~~~~~~~~~~~~~~~~~
> +
> +Since version 0.8.0 ZFS allows adding a `special` device to a pool,
which is
> +then used to store metadata, deduplication tables and optionally
small file
> +blocks.
Since version 0.8. ZFS supports `special` devices. A `special` device in
a pool is used to store metadata, deduplication tables, and optionally
small file blocks.
> +
> +IMPORTANT: The redundancy of the `special` device should match the
one of the
> +pool, since the `special` device is a point of failure for the whole
pool.
> +
> +WARNING: Adding a `special` device to a pool cannot be undone!
> +
> +.Create a pool with `special` device and RAID-1:
> +
> + zpool create -f -o ashift=12 <pool> mirror <device1> <device2>
special mirror <device3> <device4>
> +
> +.Add a `special` device to an existing pool with RAID-1:
> +
> + zpool add <pool> special mirror <device1> <device2>
> +
> +For ZFS datasets where the `special_small_blocks` property is set to
a non-zero
> +value, the `special` device is used to store small file blocks up to
that size.
> +Setting the `special_small_blocks` property on the pool will change
the default
> +value of that property for all child ZFS datasets (for example all
containers
> +in the pool will opt in for small file blocks).
> +
> +.Opt in for small file blocks pool-wide:
> +
> + zfs set special_small_blocks=<size> <pool>
> +
> +.Opt in for small file blocks for a single dataset:
> +
> + zfs set special_small_blocks=<size> <pool>/<filesystem>
> +
> +.Opt out from small file blocks for a single dataset:
> +
> + zfs set special_small_blocks=0 <pool>/<filesystem>
INFO: The value for <size> can be `0` to disable storing small file
blocks on the special device or a power of two in the range between 512B
to 128K.
Another thing I'll add here is about the (non-intuitive) relation with
the recordsize. Setting small_file_blocks higher or equal than the
recordsize of the ZFS file system will cause *all* data to be written to
the special device [0].
> +
> +Using a `special` device makes sense for pools with lots and lots of
changing
> +metadata respectively small files. If you also have other, larger
I/O on the
> +same pool then the benefit from using a `special` device might be
even more
> +noticeable. It is recommended to use SSDs or NVMes for the `special`
device.
>
A `special` device can improve the speed of small I/O operations if the
pool consists of slow spinning hard disks. Enabling
`special_small_blocks` can further increase the performance if a lot of
small files are used. Use fast (NVME) SSDs for the `special` device.
It's really about metadata and not small I/O operations in general. For
example having I/O operations with block-size 4K, but on large files
will not benefit from a special device (even with small_file_blocks
enabled).
And I think that the benefit does not depend so much on the speed of the
SSD. It should come from the fact that the I/O on the HDDs doesn't get
disturbed as much by the metadata/small file operations.
What about the following?
A `special` device can improve the speed of a pool consisting of slow
spinning hard disks with a lot of changing metadata. For example if the
pool has many short-lived files. Enabling `special_small_blocks` can
further increase the performance when those files are small. Use SSDs
for the `special` device.
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[0]: https://github.com/zfsonlinux/zfs/issues/9131#issuecomment-523680936
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