Md cache?

Md cache?

[Brian Pontius]

to all,

Is anyone aware of a way to cause the md devices to use some system memory as a write cache? (maybe through mdadm?).  I have looked everywhere and cannot seem to find anything about such a feature.

If not, anyone  know if this is a possibility for the future, Neil?

Thanks,
Brian

Re: Md cache?

[Neil Brown]

On Wednesday June 2, linuxboy123@mindspring.com wrote:
> to all,
> 
> Is anyone aware of a way to cause the md devices to use some system memory as a write cache? (maybe through mdadm?).  I have looked everywhere and cannot seem to find anything about such a feature.
> 
> If not, anyone  know if this is a possibility for the future, Neil?

What is it exactly that you want.

All device IO is cached in memory before being written to the device.
The only real value of a cache with a RAID array is if it we a
non-volatile cache.  Is that what you were thinking of.

NeilBrown

Re: Md cache?

[Daniel Pittman]

On 3 Jun 2004, Brian Pontius wrote:
> Is anyone aware of a way to cause the md devices to use some system
> memory as a write cache? (maybe through mdadm?). I have looked
> everywhere and cannot seem to find anything about such a feature.

Linux uses system memory to cache writes; adding a second layer of
caching is really quite unlikely to add anything except a second point
of failure, unless your system memory is battery backed...

   Daniel
-- 
Gratitude is one of the least articulate of the emotions, 
especially when it is deep.
        -- Felix Frankfurter

Re: Md cache?

[Brian Pontius]

I apologze for being unclear.

As I did comparisons between software and hardware raid,  one of the advantage
s that I noticed with hardware raid was cache (both read and write). With hard
ware it is usually larger giving some increase in performance.  I was looking
for a way to allocate more system memory specifically to be the md cache area
to give that extra feature of that hardware raid. This would especially be ben
efitial for a NAS that has a dedicated purpose to fast reliable storage.


Thanks for your time.
Brian Pontius


-----Original Message-----
From: Neil Brown <neilb@cse.unsw.edu.au>
Sent: Jun 2, 2004 10:10 PM
To: Brian Pontius <linuxboy123@mindspring.com>
Cc: linux-raid@vger.kernel.org
Subject: Re: Md cache?

On Wednesday June 2, linuxboy123@mindspring.com wrote:
> to all,
> 
> Is anyone aware of a way to cause the md devices to use some system memory as a write cache? (maybe through mdadm?).  I have looked everywhere and cannot seem to find anything about such a feature.
> 
> If not, anyone  know if this is a possibility for the future, Neil?

What is it exactly that you want.

All device IO is cached in memory before being written to the device.
The only real value of a cache with a RAID array is if it we a
non-volatile cache.  Is that what you were thinking of.

NeilBrown

RE: Md cache?

[Guy]

With hardware RAID the write cache has battery backup.  So, the RAID system
will not lose data when the power fails.  It will finish the writes when the
power returns.

I don't think the hardware RAID cards have a battery.  The external self
contained RAID systems that I have worked with do have batteries.

Anyone know?  Do any RAID cards have batteries?

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Brian Pontius
Sent: Thursday, June 03, 2004 2:14 PM
To: Neil Brown
Cc: linux-raid@vger.kernel.org
Subject: Re: Md cache?

I apologze for being unclear.

As I did comparisons between software and hardware raid,  one of the
advantage
s that I noticed with hardware raid was cache (both read and write). With
hard
ware it is usually larger giving some increase in performance.  I was
looking
for a way to allocate more system memory specifically to be the md cache
area
to give that extra feature of that hardware raid. This would especially be
ben
efitial for a NAS that has a dedicated purpose to fast reliable storage.


Thanks for your time.
Brian Pontius


-----Original Message-----
From: Neil Brown <neilb@cse.unsw.edu.au>
Sent: Jun 2, 2004 10:10 PM
To: Brian Pontius <linuxboy123@mindspring.com>
Cc: linux-raid@vger.kernel.org
Subject: Re: Md cache?

On Wednesday June 2, linuxboy123@mindspring.com wrote:
> to all,
> 
> Is anyone aware of a way to cause the md devices to use some system memory
as a write cache? (maybe through mdadm?).  I have looked everywhere and
cannot seem to find anything about such a feature.
> 
> If not, anyone  know if this is a possibility for the future, Neil?

What is it exactly that you want.

All device IO is cached in memory before being written to the device.
The only real value of a cache with a RAID array is if it we a
non-volatile cache.  Is that what you were thinking of.

NeilBrown

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Re: Md cache?

[Brock Nanson]

My IBM ServeRAID 3HB's do.  But as you say, many don't.  A UPS with a 
communications cable and shutdown software is really the only solution.

Brock

Guy wrote:

> With hardware RAID the write cache has battery backup.  So, the RAID system
> will not lose data when the power fails.  It will finish the writes when the
> power returns.
> 
> I don't think the hardware RAID cards have a battery.  The external self
> contained RAID systems that I have worked with do have batteries.
> 
> Anyone know?  Do any RAID cards have batteries?
> 
> Guy
> 
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Brian Pontius
> Sent: Thursday, June 03, 2004 2:14 PM
> To: Neil Brown
> Cc: linux-raid@vger.kernel.org
> Subject: Re: Md cache?
> 
> I apologze for being unclear.
> 
> As I did comparisons between software and hardware raid,  one of the
> advantage
> s that I noticed with hardware raid was cache (both read and write). With
> hard
> ware it is usually larger giving some increase in performance.  I was
> looking
> for a way to allocate more system memory specifically to be the md cache
> area
> to give that extra feature of that hardware raid. This would especially be
> ben
> efitial for a NAS that has a dedicated purpose to fast reliable storage.
> 
> 
> Thanks for your time.
> Brian Pontius
> 
> 
> -----Original Message-----
> From: Neil Brown <neilb@cse.unsw.edu.au>
> Sent: Jun 2, 2004 10:10 PM
> To: Brian Pontius <linuxboy123@mindspring.com>
> Cc: linux-raid@vger.kernel.org
> Subject: Re: Md cache?
> 
> On Wednesday June 2, linuxboy123@mindspring.com wrote:
> 
>>to all,
>>
>>Is anyone aware of a way to cause the md devices to use some system memory
> 
> as a write cache? (maybe through mdadm?).  I have looked everywhere and
> cannot seem to find anything about such a feature.
> 
>>If not, anyone  know if this is a possibility for the future, Neil?
> 
> 
> What is it exactly that you want.
> 
> All device IO is cached in memory before being written to the device.
> The only real value of a cache with a RAID array is if it we a
> non-volatile cache.  Is that what you were thinking of.
> 
> NeilBrown
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

RE: Md cache?

[Guy]

A UPS does not protect from kernel crashes!
Software RAID would lose any data not written to the disks when the kernel
crashes.
Hardware RAID does, at least to the point that data made it to the array.
I have never had a Linux kernel crash myself.

I have seen a power failure caused when a UPS failed.
To protect from UPS failure, you can have redundant UPSs.
I don't.  :(
APC has a product that allows 2 UPSs to power 1 power strip.  This gives you
redundant UPSs.  I have never used it, just read about it years ago.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Brock Nanson
Sent: Thursday, June 03, 2004 4:17 PM
To: Guy
Cc: 'Brian Pontius'; linux-raid@vger.kernel.org
Subject: Re: Md cache?

My IBM ServeRAID 3HB's do.  But as you say, many don't.  A UPS with a 
communications cable and shutdown software is really the only solution.

Brock

Guy wrote:

> With hardware RAID the write cache has battery backup.  So, the RAID
system
> will not lose data when the power fails.  It will finish the writes when
the
> power returns.
> 
> I don't think the hardware RAID cards have a battery.  The external self
> contained RAID systems that I have worked with do have batteries.
> 
> Anyone know?  Do any RAID cards have batteries?
> 
> Guy
> 
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Brian Pontius
> Sent: Thursday, June 03, 2004 2:14 PM
> To: Neil Brown
> Cc: linux-raid@vger.kernel.org
> Subject: Re: Md cache?
> 
> I apologze for being unclear.
> 
> As I did comparisons between software and hardware raid,  one of the
> advantage
> s that I noticed with hardware raid was cache (both read and write). With
> hard
> ware it is usually larger giving some increase in performance.  I was
> looking
> for a way to allocate more system memory specifically to be the md cache
> area
> to give that extra feature of that hardware raid. This would especially be
> ben
> efitial for a NAS that has a dedicated purpose to fast reliable storage.
> 
> 
> Thanks for your time.
> Brian Pontius
> 
> 
> -----Original Message-----
> From: Neil Brown <neilb@cse.unsw.edu.au>
> Sent: Jun 2, 2004 10:10 PM
> To: Brian Pontius <linuxboy123@mindspring.com>
> Cc: linux-raid@vger.kernel.org
> Subject: Re: Md cache?
> 
> On Wednesday June 2, linuxboy123@mindspring.com wrote:
> 
>>to all,
>>
>>Is anyone aware of a way to cause the md devices to use some system memory
> 
> as a write cache? (maybe through mdadm?).  I have looked everywhere and
> cannot seem to find anything about such a feature.
> 
>>If not, anyone  know if this is a possibility for the future, Neil?
> 
> 
> What is it exactly that you want.
> 
> All device IO is cached in memory before being written to the device.
> The only real value of a cache with a RAID array is if it we a
> non-volatile cache.  Is that what you were thinking of.
> 
> NeilBrown
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
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