Re: ZFS with Linux: An Open Plea

Jeff Garzik
2007-04-18T17:40:33Z

Lennart Sorensen wrote:
> On Mon, Apr 16, 2007 at 10:18:45PM +0200, Tomasz K?oczko wrote:
>> Of cources it can be true in most cases (probably for some more advanced
>> RAID controlers). Few weeks ago I perform some basic test on Dell 2950
>> with 8x73GB SAS disk .. just as for kill time (waiting for access to some
>> bigger box ;). This small iron box have inside RAID controller (Dell uses
>> in this box LSI Logic SAS MegaRAID based ctrl). Anykind combinations on
>> controler level RAID was slower than using this as plain JBOD with LVM or
>> MD+LVM. Diffrence between HW and soft RAID was not so big (1-6% depending
>> on configuration) but allways HW produces worser results (don't ask me
>> why). Finaly I decide using this disk as four RAID1 luns only because
>> under Linux I can't read each phisical disk SMART data and protecting this
>> by RAID on controller level and collecting SNMP traps from DRAC card was
>> kind of worakaround for this (in my case it will be better constanlty
>> monitor disk healt and collesting some SMART data for observe trends on
>> for example zabbix graphs for try predict some faults using triggers). On
>> top of this was configured diffrent types of volumes on LVM level (some
>> with stripping some without, some with bigger some with smaller chunk
>> size).
>
> Does it matter that google's recent report on disk failures indicated
> that SMART never predicted anything useful as far as they could tell?
> Certainly none of my drive failures ever had SMART make any kind of
> indication that anything was wrong.
>
> I think the main benefit of MD raid, is that it is portable, doesn't
> lock you into a specific piece of hardware, and you can span multiple
> controllers, and it is likely easier to have bugs in MD raid fixed that
> in some raid controller's firmware if any were to be found. Performance
> advantages are a bonus of course.

SMART largely depends on how you use it. Simply polling the current
status will not give you all the benefits SMART provides. On the
dedicated servers that I rent, running the extended test ('-t long')
often finds problems before you start losing data, or deal with a drive
death. Certainly not a huge sample size, but it backs up what I hear in
the field. Running the SMART tests on a weekly basis seems most
effective, though you'll want to stagger the tests if running in a RAID set.

Jeff