Re: write/read cache raid5

Re: write/read cache raid5

[raid]

On Mon, 8 Oct 2001, Alan Cox wrote:

> > With current memory prices i think about buying 2gb memory for my
> > fileserver (6*80gb raid5 ide)
> > Now i would like to use this memory for diskcache (write cache?)
> > Are there any things i can change to the kernel/samba to speedup things ?
>
> Linux will use free memory as caches. The one limitation you have is that
> it can't use it for write caching that much because the memory isnt
> protected, battery backed, ECC'd etc. That is one place where things like
> the DPT (now Adaptec) millenium hardware raid can do a lot better than
> software solutions

So there is no way i can Speedup write to the raid5 array ?
(memory will be ecc and the server will be on ups)

Re: write/read cache raid5

[Alistair Riddell]

On Mon, 8 Oct 2001 raid@ddx.a2000.nu wrote:

> So there is no way i can Speedup write to the raid5 array ?
> (memory will be ecc and the server will be on ups)

Your disks go as fast as they go, that is a physical limitation. 

More RAM means your server can store up data blocks to be written when the
disks are less busy. But the data still has to be written to disk
sometime.

More RAM will certainly help by caching reads though.

6 disks raided together means the bottleneck will likely be your network,
unless your server is on gigabit ethernet and has a ton of clients and/or
gigabit to the desktop.

-- 
Alistair Riddell - BOFH
IT Manager, George Watson's College, Edinburgh
Tel: +44 131 447 7931 Ext 176       Fax: +44 131 452 8594
Microsoft - because god hates us

Re: write/read cache raid5

[David Chow]

Alistair Riddell ¼g¹D¡G
> 
> On Mon, 8 Oct 2001 raid@ddx.a2000.nu wrote:
> 
> > So there is no way i can Speedup write to the raid5 array ?
> > (memory will be ecc and the server will be on ups)
> 
> Your disks go as fast as they go, that is a physical limitation.
> 
> More RAM means your server can store up data blocks to be written when the
> disks are less busy. But the data still has to be written to disk
> sometime.
> 
> More RAM will certainly help by caching reads though.
> 
> 6 disks raided together means the bottleneck will likely be your network,
> unless your server is on gigabit ethernet and has a ton of clients and/or
> gigabit to the desktop.
> 
> --
> Alistair Riddell - BOFH
> IT Manager, George Watson's College, Edinburgh
> Tel: +44 131 447 7931 Ext 176       Fax: +44 131 452 8594
> Microsoft - because god hates us
> 
> -
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Yes my server serve lots of clients and have lots of NICs even
gigabit... how can I increase write/read cache on RAID5 ? It is better
performed when big cache allows on top (before) raid computation work
and physical disk writes.

Re: write/read cache raid5

[Alistair Riddell]

On Tue, 9 Oct 2001, David Chow wrote:

> Yes my server serve lots of clients and have lots of NICs even
> gigabit... how can I increase write/read cache on RAID5 ? It is better
> performed when big cache allows on top (before) raid computation work
> and physical disk writes.

In that case more memory will certainly help. You might like to check out
Jens Axboe's block-highmem patch if you have more than 1GB of RAM. It
allows hardware to DMA direct to high memory, rather than using bounce
buffers, which can increase performance considerably.

-- 
Alistair Riddell - BOFH
IT Manager, George Watson's College, Edinburgh
Tel: +44 131 447 7931 Ext 176       Fax: +44 131 452 8594
Microsoft - because god hates us

Re: write/read cache raid5

[Alan Cox]

> > protected, battery backed, ECC'd etc. That is one place where things like
> > the DPT (now Adaptec) millenium hardware raid can do a lot better than
> > software solutions
> 
> So there is no way i can Speedup write to the raid5 array ?
> (memory will be ecc and the server will be on ups)

And you have no ECC on the PCI bus, nor will a UPS protect against a crash.

Re: write/read cache raid5

[Nico Dummer]

Alan Cox wrote:

>>>protected, battery backed, ECC'd etc. That is one place where things like
>>>the DPT (now Adaptec) millenium hardware raid can do a lot better than
>>>software solutions

>>So there is no way i can Speedup write to the raid5 array ?
>>(memory will be ecc and the server will be on ups)

> And you have no ECC on the PCI bus, nor will a UPS protect against a crash.

You have this Problem anyway with or without Writecaching, the only 
difference is that it will exist longer with Writecaching turned on than 
without it.

Something to think about:

A possible Solution for this Problem is to get 2 Raid5 Arrays, a Primary 
Array for fast writing and a Secondary for fast reading.
All incoming Data will be written to a Rambuffer and the Primary Array 
while Reads are handled by the Secondary (or by the Rambuffer for Data 
not written to the Secondary). In times with fewer load on the Secondary 
Array the Data can be written from Rambuffer to it.
The Data is save at the Point where it´s on the Primary Array while you 
have unblocked Performance for Reads, in case of a crash the Secondary 
Array need to be synced with the Primary.
A list with unsynced blocks can replace the Writebuffer to take more 
Cachebuffers for Reads and so reduce load on the Secondary Array that 
can be used for syncing.
You will get the Security of 1 Raid5 Array if the Data is unsynced and 
the Security of a mirrored Raid5 Array during low load times so the 
Money wont be completely wasted only for Performance.
Would be nice if you guys consider Writecaching as an Option for this 
configuration too if most possible Security isnt needed or can be low 
for a few Seconds between buffering and writing. Let the People choose, 
they will do the right decision for themselves.

write/read cache raid5

[raid]

With current memory prices i think about buying 2gb memory for my
fileserver (6*80gb raid5 ide)
Now i would like to use this memory for diskcache (write cache?)
Are there any things i can change to the kernel/samba to speedup things ?

filesystem is shared using nfs and samba (using a gigabit card)