Controlling Reading and Writing in RAID 1 configuration

Controlling Reading and Writing in RAID 1 configuration

[Rik Herrin]

Hi,
  I have a setup where I want to use RAID 1 to mirror
my data over the LAN.  I am exposing 2x500 GB-sized
devices via iSCSI and was wondering if I could
configure the RAID 1 to write to both members of the
RAID 1 configuration but only read from one as one
link is 1GbE while the other is lower but sufficient
for writing.  Is this possible? Thanks for your help.

Regards,
  Rik Herrin

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Re: Controlling Reading and Writing in RAID 1 configuration

[Neil Brown]

On Tuesday April 25, rikherrin@yahoo.com wrote:
> Hi,
>   I have a setup where I want to use RAID 1 to mirror
> my data over the LAN.  I am exposing 2x500 GB-sized
> devices via iSCSI and was wondering if I could
> configure the RAID 1 to write to both members of the
> RAID 1 configuration but only read from one as one
> link is 1GbE while the other is lower but sufficient
> for writing.  Is this possible? Thanks for your help.

You'll be wanting the '--write-mostly' option to mdadm.
This requires a fairly recent kernel and mdadm.

NeilBrown

Re: Controlling Reading and Writing in RAID 1 configuration

[Ming Zhang]

On Fri, 2006-04-28 at 08:04 +1000, Neil Brown wrote:
> On Tuesday April 25, rikherrin@yahoo.com wrote:
> > Hi,
> >   I have a setup where I want to use RAID 1 to mirror
> > my data over the LAN.  I am exposing 2x500 GB-sized
> > devices via iSCSI and was wondering if I could
> > configure the RAID 1 to write to both members of the
> > RAID 1 configuration but only read from one as one

only read from one disk, shall this be a "--read-preferred" ?

i checked email archive, but still not quite understand on this
"--write-mostly". for raid1, u anyway need to write all components, so
why there is a "mostly"?


> > link is 1GbE while the other is lower but sufficient
> > for writing.  Is this possible? Thanks for your help.
> 
> You'll be wanting the '--write-mostly' option to mdadm.
> This requires a fairly recent kernel and mdadm.
> 
> NeilBrown
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Re: Controlling Reading and Writing in RAID 1 configuration

[Neil Brown]

On Thursday April 27, mingz@ele.uri.edu wrote:
> 
> On Fri, 2006-04-28 at 08:04 +1000, Neil Brown wrote:
> > On Tuesday April 25, rikherrin@yahoo.com wrote:
> > > Hi,
> > >   I have a setup where I want to use RAID 1 to mirror
> > > my data over the LAN.  I am exposing 2x500 GB-sized
> > > devices via iSCSI and was wondering if I could
> > > configure the RAID 1 to write to both members of the
> > > RAID 1 configuration but only read from one as one
> 
> only read from one disk, shall this be a "--read-preferred" ?

No.  The one(s) you don't read from is the odd-one-out.  The one(s)
you read from are 'normal' and don't get a special flag.

> 
> i checked email archive, but still not quite understand on this
> "--write-mostly". for raid1, u anyway need to write all components, so
> why there is a "mostly"?
> 

Mostly it will just write to this device.
But if all the non-write-mostly device fail, then it will read from
this device.  So it isn't write-only.  It is write-mostly.

Make sense?

NeilBrown

Re: Controlling Reading and Writing in RAID 1 configuration

[Ming Zhang]

On Fri, 2006-04-28 at 09:06 +1000, Neil Brown wrote:
> On Thursday April 27, mingz@ele.uri.edu wrote:
> > 
> > On Fri, 2006-04-28 at 08:04 +1000, Neil Brown wrote:
> > > On Tuesday April 25, rikherrin@yahoo.com wrote:
> > > > Hi,
> > > >   I have a setup where I want to use RAID 1 to mirror
> > > > my data over the LAN.  I am exposing 2x500 GB-sized
> > > > devices via iSCSI and was wondering if I could
> > > > configure the RAID 1 to write to both members of the
> > > > RAID 1 configuration but only read from one as one
> > 
> > only read from one disk, shall this be a "--read-preferred" ?
> 
> No.  The one(s) you don't read from is the odd-one-out.  The one(s)
> you read from are 'normal' and don't get a special flag.
> 
> > 
> > i checked email archive, but still not quite understand on this
> > "--write-mostly". for raid1, u anyway need to write all components, so
> > why there is a "mostly"?
> > 
> 
> Mostly it will just write to this device.
> But if all the non-write-mostly device fail, then it will read from
> this device.  So it isn't write-only.  It is write-mostly.
> 
> Make sense?

i know why my English language test had that low score now.

thx!



> 
> NeilBrown