Thread: Battery Backed Cache for RAID

Battery Backed Cache for RAID

From
"Peter Darley"
Date:
Folks,
    I'm getting a new server for our database, and I have a quick question
about RAID controllers with a battery backed cache.  I understand that the
cache will allow the cache to be written out if the power fails to the box,
which allows it to report a write as committed safely when it's not actually
committed.
    My question is, if the power goes off, and the drives stop, how does the
battery backed cache save things out to the dead drives?  Is there another
component that is implied that will provide power to the drives that I
should be looking into as well?
Thanks,
Peter Darley


Re: Battery Backed Cache for RAID

From
"Jeffrey W. Baker"
Date:
On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
>     I'm getting a new server for our database, and I have a quick question
> about RAID controllers with a battery backed cache.  I understand that the
> cache will allow the cache to be written out if the power fails to the box,
> which allows it to report a write as committed safely when it's not actually
> committed.

Actually the cache will just hold its contents while the power is out.
When the power is restored, the RAID controller will complete the writes
to disk.  If the battery does not last through the outage, the data is
lost.

>     My question is, if the power goes off, and the drives stop, how does the
> battery backed cache save things out to the dead drives?  Is there another
> component that is implied that will provide power to the drives that I
> should be looking into as well?

A UPS would allow you to do an orderly shutdown and write contents to
disk during a power failure.  However a UPS can be an extra point of
failure.

-jwb

Re: Battery Backed Cache for RAID

From
Alvaro Herrera
Date:
On Wed, Sep 14, 2005 at 11:28:43AM -0700, Jeffrey W. Baker wrote:
> On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
> >     I'm getting a new server for our database, and I have a quick question
> > about RAID controllers with a battery backed cache.  I understand that the
> > cache will allow the cache to be written out if the power fails to the box,
> > which allows it to report a write as committed safely when it's not actually
> > committed.
>
> Actually the cache will just hold its contents while the power is out.
> When the power is restored, the RAID controller will complete the writes
> to disk.  If the battery does not last through the outage, the data is
> lost.

Just curious: how long are the batteries supposed to last?

--
Alvaro Herrera -- Valdivia, Chile         Architect, www.EnterpriseDB.com
Hi! I'm a .signature virus!
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Re: Battery Backed Cache for RAID

From
"Merlin Moncure"
Date:
> On Wed, Sep 14, 2005 at 11:28:43AM -0700, Jeffrey W. Baker wrote:
> > On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
> > >     I'm getting a new server for our database, and I have a quick
> question
> > > about RAID controllers with a battery backed cache.  I understand
that
> the
> > > cache will allow the cache to be written out if the power fails to
the
> box,
> > > which allows it to report a write as committed safely when it's
not
> actually
> > > committed.
> >
> > Actually the cache will just hold its contents while the power is
out.
> > When the power is restored, the RAID controller will complete the
writes
> > to disk.  If the battery does not last through the outage, the data
is
> > lost.
>
> Just curious: how long are the batteries supposed to last?

For the length of time it will take for you to get fired for not getting
the server running plus one hour :).

Merlin

Re: Battery Backed Cache for RAID

From
John A Meinel
Date:
Alvaro Herrera wrote:
> On Wed, Sep 14, 2005 at 11:28:43AM -0700, Jeffrey W. Baker wrote:
>
>>On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
>>
>>>    I'm getting a new server for our database, and I have a quick question
>>>about RAID controllers with a battery backed cache.  I understand that the
>>>cache will allow the cache to be written out if the power fails to the box,
>>>which allows it to report a write as committed safely when it's not actually
>>>committed.
>>
>>Actually the cache will just hold its contents while the power is out.
>>When the power is restored, the RAID controller will complete the writes
>>to disk.  If the battery does not last through the outage, the data is
>>lost.
>
>
> Just curious: how long are the batteries supposed to last?
>

The recent *cheap* version of a ramdisk had battery backup for 16 hours.
(Very expensive ramdisks actually have enough battery power to power a
small hard-drive to dump the contents into).

I'm guessing for a RAID controller, the time would be in the max 1 day
range.

John
=:->


Attachment

Re: Battery Backed Cache for RAID

From
"Welty, Richard"
Date:
John A Meinel wrote:
>The recent *cheap* version of a ramdisk had battery backup for 16 hours.
>(Very expensive ramdisks actually have enough battery power to power a
>small hard-drive to dump the contents into).

>I'm guessing for a RAID controller, the time would be in the max 1 day
>range.

i think some will go a bit longer. i have seen an IBM ServeRaid (rebranded
mylex in this particular case) keep its memory after being pulled for a
remarkably long period of time.

no guarantees, though, so i'm not actually going to say how long so that nobody
gets unreasonable expectations.

richard

Re: Battery Backed Cache for RAID

From
Arjen van der Meijden
Date:
On 14-9-2005 22:03, Alvaro Herrera wrote:
> On Wed, Sep 14, 2005 at 11:28:43AM -0700, Jeffrey W. Baker wrote:
>
>>On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
>>
>>Actually the cache will just hold its contents while the power is out.
>>When the power is restored, the RAID controller will complete the writes
>>to disk.  If the battery does not last through the outage, the data is
>>lost.
>
>
> Just curious: how long are the batteries supposed to last?

For the LSI-Logic MegaRaid 320-2e its about 72 hours for the standard
128MB version. Their SATA2-solution offers 32 and 72 hour-options. So I
assume its "in the order of days" for most RAID controllers.

Best regards,

Arjen van der Meijden

Re: Battery Backed Cache for RAID

From
mudfoot@rawbw.com
Date:
Bear in mind you will lose data if the raid controller itself fails (or the
cache memory module).  Many solutions have mirrored cache for this reason.  But
that's more $$, depending on the risks you want to take.

Quoting Arjen van der Meijden <acmmailing@tweakers.net>:

> On 14-9-2005 22:03, Alvaro Herrera wrote:
> > On Wed, Sep 14, 2005 at 11:28:43AM -0700, Jeffrey W. Baker wrote:
> >
> >>On Wed, 2005-09-14 at 11:25 -0700, Peter Darley wrote:
> >>
> >>Actually the cache will just hold its contents while the power is out.
> >>When the power is restored, the RAID controller will complete the writes
> >>to disk.  If the battery does not last through the outage, the data is
> >>lost.
> >
> >
> > Just curious: how long are the batteries supposed to last?
>
> For the LSI-Logic MegaRaid 320-2e its about 72 hours for the standard
> 128MB version. Their SATA2-solution offers 32 and 72 hour-options. So I
> assume its "in the order of days" for most RAID controllers.
>
> Best regards,
>
> Arjen van der Meijden
>
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