[EMAIL PROTECTED] wrote:
Robert,
I don't understand why not loosing any data is an advantage of ZFS.
No filesystem should lose any data. It is like saying that an advantage
of football player is that he/she plays football (he/she should do that !)
or an advantage of chef is that he/she cooks (he/she should do that !).
Every filesystem should _save_ our data, not lose it.
yes, you are right: every filesystem should save the data.
(... and every program should have no error! ;-)
Unfortunately there are some cases, where the disks lose data,
these cannot be detected by traditional filesystems but with ZFS:
* bit rot: some bits on the disk gets flipped (~ 1 in 10^11)
(cosmic rays, static particles in airflow, random thermodynamics)
* phantom writes: a disk 'forgets' to write data (~ 1 in 10^8)
(positioning errors, disk firmware errors, ...)
* misdirected reads/writes: disk writes to the wrong position (~ 1
in 10^8)
(disks use very small structures, head can move after positioning)
* errors on the data transfer connection
You can look up the probabilities at several disk vendors, the are
published.
Traditional filesystems do not check the data they read. You get strange
effects
when the filesystem code runs with wrong metadata (worst case: panic).
If you use the wrong data in your applicaton, you 'only' have the wrong
results...
ZFS on the contrary checks every block it reads and is able to find the
mirror
or reconstruct the data in a raidz config.
Therefore ZFS uses only valid data and is able to repair the data blocks
automatically.
This is not possible in a traditional filesystem/volume manager
configuration.
You may say, you never heard of a disk losing data; but you have heard
of systems,
which behave strange and a re-installation fixed everything.
Or some data have gone bad and you have to recover from backup.
It may be, that this was one of these cases.
Our service encounters a number of these cases every year,
where the customer was not able to re-install or did not want to restore
his data,
which can be traced back to such a disk error.
These are always nasty problems and it gets nastier, because customers
have more and more data and there is a trend to save money on backup/restore
infrastructures which make it hurt to restore data.
Regards,
Ulrich
--
| Ulrich Graef, Senior Consultant, OS Ambassador \
| Operating Systems, Performance \ Platform Technology \
| Mail: [EMAIL PROTECTED] \ Global Systems Enginering \
| Phone: +49 6103 752 359 \ Sun Microsystems Inc \
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