Steve Hull wrote:
> Sooo... I've been reading a lot in various places. The conclusion I've
> drawn is this:
>
> I can create raidz vdevs in groups of 3 disks and add them to my zpool to be
> protected against 1 drive failure. This is the current status of "growing
> protected space" in raidz. Am I correct here?
>
>
> This message posted from opensolaris.org
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Correct. Here's some quick summary information:
a POOL is made of 1 or more VDEVs.
POOLs consisting of more than 1 VDEV will stripe data across all the VDEVs.
VDEVS may be freely added to any POOL, but cannot currently be removed
from a POOL.
When a vdev is added to a pool, data on the existing vdevs is not
automatically re-distributed. That is, say you have 3 vdevs of 1GB each,
and add another vdev of 1GB. The system does not immediately attempt
to re-distribute the data on the original 3 devices. It will re-balance
the data as you WRITE to the pool. Thus, if you expand a pool like this,
it is a good idea to copy the data around. i.e.
cp /zpool/olddir /zpool/newdir
rm -rf /zpool/olddir
If there are more than 1 vdev in a pool, the pool's capacity is
determined by the smallest device. Thus, if you have a 2GB, a 3GB, and a
5GB device in a pool, the pool's capacity is 3 x 2GB = 6GB, as ZFS will
only do full-stripes. Thus, there really is no equivalent to
Concatenation in other RAID solutions.
However, if you replace ALL devices in a pool with larger ones, ZFS will
automatically expand the pool size. Thus, if you replaced the 2GB
devices in the above case with 4GB devices, then the pool would
automatically appear to be 3 x 4GB = 12GB.
A VDEV can consist of:
any file
any disk slice/partition
a whole disk (preferred!)
a special sub-device, raidz/raidz1/raidz2/mirror/cache/log/spare
For the special sub-devices, here's a summary:
raidz (synonym raidz1):
You must provide at LEAST 3 storage devices (where a file, slice,
or disk is a storage device)
1 device's capacity is consumed in parity.
However, parity is scattered around the devices, thus this is
roughly analogous to RAID-5
Currently, devices CANNOT be added or removed from a raidz.
It is possible to increase the size of raidz by replacing each
drive, ONE AT A TIME, with a larger drive. But altering the NUMBER of
drives is not possible.
raidz2:
You must have at LEAST _4_ storage devices
2 device's capacity is consumed by parity.
Like raidz, parity is scattered around the devices, improving I/O
performance.
Roughly analogous to RAID-6.
Altering a raidz2 is exactly like doing a raidz.
mirror
You must provide at LEAST 2 storage devices
All data is replicated across all devices, acting as a "normal"
mirror.
You can add or detach devices from a mirror at will, so long as
they are at least a big as the original mirror.
spare
Indicates a device which can be used as a hot spare.
log
indicates an Intent Log, which is basically a transactional log
of filesystem operations.
Generally speaking, this is used only for certain
high-performance cases, and tends to be
used in association with enterprise-level devices, such as
solid-state drives.
cache
similar to an Intent Log, this provide a place to cache
filesystem internals (metadata such as directory/file attributes)
usually used in situations similar to log devices.
--------
All pools store redundant metadata, so they can automatically detect
and repair most faults in metadata.
If you vdev is raidz, raidz2 or mirror, they store redundant data
(which allows them to recover from losing a disk), so they can
automatically detect AND repair block-level faults.
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
Timezone: US/Pacific (GMT-0800)
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