write only raid1

write only raid1

[Ming Zhang]

sorry that i must missed u reply.

is there any technical issue that lead not to be implemented?

is there any place to find previous discussion on this? i try the google
but fail. i do no know which key word can find that. thx.

ming

On Thursday November 4, mingz@ele.uri.edu wrote:
> Hi, folks?
> 
> I would like to know if current RAID1 support or willing to support a
> RAID1 with a write only device? For example, u can setup 2 disks to
form
> a raid1, you fork 2 writes to both disks while you only read from one.
> this will be very useful for logging purpose or replication.

This functionality has been discussed, but have not been implemented
in a released kernel yet.

NeilBrown

Re: write only raid1

[Paul Clements]

Ming Zhang wrote:

> is there any technical issue that lead not to be implemented?

No. In fact, it has been implemented. Both Peter T. Breuer and I have 
come up with slightly different patches to implement the write only 
functionality.

> is there any place to find previous discussion on this? i try the google
> but fail. i do no know which key word can find that. thx.

Try the archives of this mailing list:

http://marc.theaimsgroup.com/?l=linux-raid&r=1&w=2

This has been discussed a couple of times and patches have been posted 
to the list in the past.

--
Paul



> On Thursday November 4, mingz@ele.uri.edu wrote:
> 
>>Hi, folks?
>>
>>I would like to know if current RAID1 support or willing to support a
>>RAID1 with a write only device? For example, u can setup 2 disks to
> 
> form
> 
>>a raid1, you fork 2 writes to both disks while you only read from one.
>>this will be very useful for logging purpose or replication.
> 
> 
> This functionality has been discussed, but have not been implemented
> in a released kernel yet.
> 
> 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
> 

Re: write only raid1

[Ming Zhang]

On Wed, 2004-11-10 at 10:05, Paul Clements wrote:
> Ming Zhang wrote:
> 
> > is there any technical issue that lead not to be implemented?
> 
> No. In fact, it has been implemented. Both Peter T. Breuer and I have 
> come up with slightly different patches to implement the write only 
> functionality.
> 
can u send me the patch then i can have a look?

> > is there any place to find previous discussion on this? i try the google
> > but fail. i do no know which key word can find that. thx.
> 
> Try the archives of this mailing list:
> 
> http://marc.theaimsgroup.com/?l=linux-raid&r=1&w=2
> 
> This has been discussed a couple of times and patches have been posted 
> to the list in the past.
> 
i must be an idiot. i search again and can not find. even with either of
your two guys' name. :P i have to change my eyeglass again. sigh.

> --
> Paul
> 
> 
> 
> > On Thursday November 4, mingz@ele.uri.edu wrote:
> > 
> >>Hi, folks?
> >>
> >>I would like to know if current RAID1 support or willing to support a
> >>RAID1 with a write only device? For example, u can setup 2 disks to
> > 
> > form
> > 
> >>a raid1, you fork 2 writes to both disks while you only read from one.
> >>this will be very useful for logging purpose or replication.
> > 
> > 
> > This functionality has been discussed, but have not been implemented
> > in a released kernel yet.
> > 
> > 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
> > 
-- 
 --------------------------------------------------
| Ming Zhang, PhD. Student
| Dept. of Electrical & Computer Engineering
| College of Engineering
| University of Rhode Island
| Kingston RI. 02881
| e-mail: mingz at ele.uri.edu
| Tel. (401) 874-2293 
| Fax. (401) 782-6422
| http://www.ele.uri.edu/~mingz/
| http://crab.ele.uri.edu/gallery/albums.php
 --------------------------------------------------

Re: write only raid1

[Paul Clements]

Ming Zhang wrote:
> On Wed, 2004-11-10 at 10:05, Paul Clements wrote:
> 
>>Ming Zhang wrote:
>>
>>
>>>is there any technical issue that lead not to be implemented?
>>
>>No. In fact, it has been implemented. Both Peter T. Breuer and I have 
>>come up with slightly different patches to implement the write only 
>>functionality.
>>
> 
> can u send me the patch then i can have a look?

my patches are here:

http://parisc-linux.org/~jejb/md_bitmap/

you're looking for the async write portion of the code, which has the 
write-only functionality in it (later patches do not contain that code, 
so look at the older ones)

check the mailing list archives for discussion

Peter's patch is here:

http://marc.theaimsgroup.com/?l=linux-raid&m=109238533827986&w=2


--
Paul

Re: write only raid1

[Ming Zhang]

i now understand what you mean. i saw that async write operation and i
do not realize that is what u think write only.

so base on the patch, since i already know exactly that one device will
never be read. a simpler way might be

1) modify mdadm to allow one device to be configured as write only
2) add a write only flag in device info and check the flag when do read
balance.

so no need to update the time at all if no persistent bitmap purpose.

i saw the 2.4.x kernel raid has write only flag while dropped in 2.6.x.
i guess that is for reconstruction. but we can make use of that for this
purpose as well.

"write only" can be part of the async write or async replication. but i
think raid1 code should be able to allow write only to be a standalone
feature so others may have their own async write solution.

ming



On Wed, 2004-11-10 at 10:56, Paul Clements wrote:
> Ming Zhang wrote:
> > On Wed, 2004-11-10 at 10:05, Paul Clements wrote:
> > 
> >>Ming Zhang wrote:
> >>
> >>
> >>>is there any technical issue that lead not to be implemented?
> >>
> >>No. In fact, it has been implemented. Both Peter T. Breuer and I have 
> >>come up with slightly different patches to implement the write only 
> >>functionality.
> >>
> > 
> > can u send me the patch then i can have a look?
> 
> my patches are here:
> 
> http://parisc-linux.org/~jejb/md_bitmap/
> 
> you're looking for the async write portion of the code, which has the 
> write-only functionality in it (later patches do not contain that code, 
> so look at the older ones)
> 
> check the mailing list archives for discussion
> 
> Peter's patch is here:
> 
> http://marc.theaimsgroup.com/?l=linux-raid&m=109238533827986&w=2
> 
> 
> --
> Paul