Here is a proposal for a new 'copies' property which would allow
different levels of replication for different filesystems.
Your comments are appreciated!
--matt
A. INTRODUCTION
ZFS stores multiple copies of all metadata. This is accomplished by
storing up to three DVAs (Disk Virtual Addresses) in each block pointer.
This feature is known as "Ditto Blocks". When possible, the copies are
stored on different disks.
See bug 6410698 "ZFS metadata needs to be more highly replicated (ditto
blocks)" for details on ditto blocks.
This case will extend this feature to allow system administrators to
store multiple copies of user data as well, on a per-filesystem basis.
These copies are in addition to any redundancy provided at the pool
level (mirroring, raid-z, etc).
B. DESCRIPTION
A new property will be added, 'copies', which specifies how many copies
of the given filesystem will be stored. Its value must be 1, 2, or 3.
Like other properties (eg. checksum, compression), it only affects
newly-written data. As such, it is recommended that the 'copies'
property be set at filesystem-creation time
(eg. 'zfs create -o copies=2 pool/fs').
The pool must be at least on-disk version 2 to use this feature (see
'zfs upgrade').
By default (copies=1), only two copies of most filesystem metadata are
stored. However, if we are storing multiple copies of user data, then 3
copies (the maximum) of filesystem metadata will be stored.
This feature is similar to using mirroring, but differs in several
important ways:
* Different filesystems in the same pool can have different numbers of
copies.
* The storage configuration is not constrained as it is with mirroring
(eg. you can have multiple copies even on a single disk).
* Mirroring offers slightly better performance, because only one DVA
needs to be allocated.
* Mirroring offers slightly better redundancy, because one disk from
each mirror can fail without data loss.
It is important to note that the copies provided by this feature are in
addition to any redundancy provided by the pool configuration or the
underlying storage. For example:
* In a pool with 2-way mirrors, a filesystem with copies=1 (the default)
will be stored with 2 * 1 = 2 copies. The filesystem can tolerate any
1 disk failing without data loss.
* In a pool with 2-way mirrors, a filesystem with copies=3
will be stored with 2 * 3 = 6 copies. The filesystem can tolerate any
5 disks failing without data loss (assuming that there are at least
ncopies=3 mirror groups).
* In a pool with single-parity raid-z a filesystem with copies=2
will be stored with 2 copies, each copy protected by its own parity
block. The filesystem can tolerate any 3 disks failing without data
loss (assuming that there are at least ncopies=2 raid-z groups).
C. MANPAGE CHANGES
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they are inherited.
+ copies=1 | 2 | 3
+ Controls the number of copies of data stored for this dataset.
+ These copies are in addition to any redundancy provided by the
+ pool (eg. mirroring or raid-z). The copies will be stored on
+ different disks if possible.
+
+ Changing this property only affects newly-written data.
+ Therefore, it is recommended that this property be set at
+ filesystem creation time, using the '-o copies=' option.
+
+
Temporary Mountpoint Properties
When a file system is mounted, either through mount(1M) for
legacy mounts or the "zfs mount" command for normal file
D. REFERENCES
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