Documentation on device-mapper and friends
I'm looking for information on device-mapper, the kernel space utility for Logical Volume Management (LVM2). It seems that the relevant code resides under drivers/md and a lot of other information is under Documentation/device-mapper/ That's fine, but is there any other document that gives more finer details about the device-mapper. I've also recently subsribed to the device-mapper mailing list. -Amit
Kumar Amit Mehta <gmate.amit@gmail.com> wrote:
I'm looking for information on device-mapper, the kernel space utility for Logical Volume Management (LVM2). It seems that the relevant code resides under drivers/md and a lot of other information is under Documentation/device-mapper/ That's fine, but is there any other document that gives more finer details about the device-mapper. I've also recently subsribed to the device-mapper mailing list.
-Amit
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html Dm is just a single block in it, but it can help to see where it fits in overall. Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems. Greg -- Sent from my Android phone with K-9 Mail. Please excuse my brevity.
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
On Mon, Apr 29, 2013 at 12:51 PM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
I actually assumed others would step in. I haven't seen any dm docs outside the kernel tree, but I haven't done and dm work. I guess you know dm-cache just went into the kernel: http://lwn.net/Articles/540996/ The primary use case is to use SSD (or similar fast storage) as a cache for a rotating disk. I assume you found this very old (and simple) article, but I doubt it is useful 8 years later: http://lwn.net/Articles/124703/ For more useful links, I often go to wikipedia and follow the links: You should find: multi-path faq: http://christophe.varoqui.free.fr/faq.html multi-path policies: http://christophe.varoqui.free.fr/policies.html multi-path diagrams: http://christophe.varoqui.free.fr/graphics/ multi-path reference: http://christophe.varoqui.free.fr/refbook.html Greg
Hi On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
Some docs about multipath can be found here http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide http://christophe.varoqui.free.fr/refbook.html The userspace part for tools is here http://sourceware.org/lvm2/
Hi Amit, I had compiled some notes on my blog. Here are some links on writing your own device mapper target. http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html Concept of device mapper target. http://techgmm.blogspot.in/p/device-mapper-layer-explored-every.html Thanks, Gaurav. On Tue, Apr 30, 2013 at 5:05 AM, Anatol Pomozov <anatol.pomozov@gmail.com>wrote:
Hi
On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
Some docs about multipath can be found here
http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide http://christophe.varoqui.free.fr/refbook.html
The userspace part for tools is here http://sourceware.org/lvm2/
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Hi Gaurav, I went through your blog and it is really informative. But after reading that i realized that i have a question: If I want to write a block device driver which is going to sit on lvm (and do some functionality on top of it) then should i go for the block device driver api or write it as a device mapper target. What are the advantages/disadvantages of both the approaches. Regards, Neha On Tue, Apr 30, 2013 at 4:24 AM, Gaurav Mahajan <gauravmahajan2007@gmail.com
wrote:
Hi Amit,
I had compiled some notes on my blog. Here are some links on writing your own device mapper target. http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html
Concept of device mapper target. http://techgmm.blogspot.in/p/device-mapper-layer-explored-every.html
Thanks, Gaurav.
On Tue, Apr 30, 2013 at 5:05 AM, Anatol Pomozov <anatol.pomozov@gmail.com>wrote:
Hi
On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
Some docs about multipath can be found here
http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide http://christophe.varoqui.free.fr/refbook.html
The userspace part for tools is here http://sourceware.org/lvm2/
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Hi Neha, LVM uses device mapper. Advantages of using device mapper is that you can stack different dm-targets on each other. I am really not aware of block device drivers. May be Greg can help us understand the actual pros and cons. Thanks, Gaurav On Wed, May 1, 2013 at 9:45 PM, neha naik <nehanaik27@gmail.com> wrote:
Hi Gaurav, I went through your blog and it is really informative. But after reading that i realized that i have a question: If I want to write a block device driver which is going to sit on lvm (and do some functionality on top of it) then should i go for the block device driver api or write it as a device mapper target. What are the advantages/disadvantages of both the approaches.
Regards, Neha
On Tue, Apr 30, 2013 at 4:24 AM, Gaurav Mahajan < gauravmahajan2007@gmail.com> wrote:
Hi Amit,
I had compiled some notes on my blog. Here are some links on writing your own device mapper target. http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html
Concept of device mapper target. http://techgmm.blogspot.in/p/device-mapper-layer-explored-every.html
Thanks, Gaurav.
On Tue, Apr 30, 2013 at 5:05 AM, Anatol Pomozov <anatol.pomozov@gmail.com
wrote:
Hi
On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer < greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
Some docs about multipath can be found here
http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide http://christophe.varoqui.free.fr/refbook.html
The userspace part for tools is here http://sourceware.org/lvm2/
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
The block layers can be layered both ways. DM is the newer infrastructure and was created in the early days of 2.6 If what I was writing could fit into a dm-target, that is what I would do. There are significant projects like drbd and mdraid that are not dm-targets, but I think their is a long term goal to incorporate mdraid's functionality at a minimum into dm. I doubt drbd is ever moved to dm. It is just too big of a project and in use in lots of production server environments. Greg On Tue, May 7, 2013 at 1:46 AM, Gaurav Mahajan <gauravmahajan2007@gmail.com> wrote:
Hi Neha,
LVM uses device mapper. Advantages of using device mapper is that you can stack different dm-targets on each other. I am really not aware of block device drivers.
May be Greg can help us understand the actual pros and cons.
Thanks, Gaurav
On Wed, May 1, 2013 at 9:45 PM, neha naik <nehanaik27@gmail.com> wrote:
Hi Gaurav, I went through your blog and it is really informative. But after reading that i realized that i have a question: If I want to write a block device driver which is going to sit on lvm (and do some functionality on top of it) then should i go for the block device driver api or write it as a device mapper target. What are the advantages/disadvantages of both the approaches.
Regards, Neha
On Tue, Apr 30, 2013 at 4:24 AM, Gaurav Mahajan <gauravmahajan2007@gmail.com> wrote:
Hi Amit,
I had compiled some notes on my blog. Here are some links on writing your own device mapper target. http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html
Concept of device mapper target. http://techgmm.blogspot.in/p/device-mapper-layer-explored-every.html
Thanks, Gaurav.
On Tue, Apr 30, 2013 at 5:05 AM, Anatol Pomozov <anatol.pomozov@gmail.com> wrote:
Hi
On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it fits in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
Some docs about multipath can be found here
http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide http://christophe.varoqui.free.fr/refbook.html
The userspace part for tools is here http://sourceware.org/lvm2/
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Hi Greg, Thanks for the information. I have another question :). Is there some less flexibility if we use device mapper target? For example in block device driver you can use the api such that it won't use the OS io scheduler, so the io comes directly to the block device driver through the 'make_request' call. With the device mapper i don't think that happens(looking at the api calls). Does this mean that stuff like io scheduling. barrier control etc is done by the device mapper itself and we can focus only on 'mapping' the io. Regards, Neha On Wed, May 8, 2013 at 8:11 AM, Greg Freemyer <greg.freemyer@gmail.com>wrote:
The block layers can be layered both ways. DM is the newer infrastructure and was created in the early days of 2.6
If what I was writing could fit into a dm-target, that is what I would do.
There are significant projects like drbd and mdraid that are not dm-targets, but I think their is a long term goal to incorporate mdraid's functionality at a minimum into dm. I doubt drbd is ever moved to dm. It is just too big of a project and in use in lots of production server environments.
Greg
On Tue, May 7, 2013 at 1:46 AM, Gaurav Mahajan <gauravmahajan2007@gmail.com> wrote:
Hi Neha,
LVM uses device mapper. Advantages of using device mapper is that you can stack different dm-targets on each other. I am really not aware of block device drivers.
May be Greg can help us understand the actual pros and cons.
Thanks, Gaurav
On Wed, May 1, 2013 at 9:45 PM, neha naik <nehanaik27@gmail.com> wrote:
Hi Gaurav, I went through your blog and it is really informative. But after
reading
that i realized that i have a question: If I want to write a block device driver which is going to sit on lvm (and do some functionality on top of it) then should i go for the block device driver api or write it as a device mapper target. What are the advantages/disadvantages of both the approaches.
Regards, Neha
On Tue, Apr 30, 2013 at 4:24 AM, Gaurav Mahajan <gauravmahajan2007@gmail.com> wrote:
Hi Amit,
I had compiled some notes on my blog. Here are some links on writing your own device mapper target.
http://techgmm.blogspot.in/p/writing-your-own-device-mapper-target.html
Concept of device mapper target. http://techgmm.blogspot.in/p/device-mapper-layer-explored-every.html
Thanks, Gaurav.
On Tue, Apr 30, 2013 at 5:05 AM, Anatol Pomozov <anatol.pomozov@gmail.com> wrote:
Hi
On Mon, Apr 29, 2013 at 9:51 AM, amit mehta <gmate.amit@gmail.com> wrote:
On Sun, Apr 28, 2013 at 5:24 PM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
A nice diagram of the overall storage subsystem is at http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram.html
Dm is just a single block in it, but it can help to see where it
fits
in overall.
Btw: that diagram doesn't show the legacy ata driver that creates /dev/hdx style devices. Has that been dropped while I wasn't paying attention? I haven't used it in years, but I thought it was still used on embedded systems.
Thank you for sharing the link, but I'm looking for more detailed information on I/O stack in Linux, dm-mapper and multipath in particular.
Some docs about multipath can be found here
http://www.sourceware.org/lvm2/wiki/MultipathUsageGuide http://christophe.varoqui.free.fr/refbook.html
The userspace part for tools is here http://sourceware.org/lvm2/
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
_______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
You should take this to the device mapper list, but I'll try here. For lurkers, this drawing may be helpful: http://www.thomas-krenn.com/en/oss/linux-io-stack-diagram/linux-io-stack-dia... On Wed, May 8, 2013 at 11:15 AM, neha naik <nehanaik27@gmail.com> wrote:
Hi Greg, Thanks for the information. I have another question :). Is there some less flexibility if we use device mapper target? For example in block device driver you can use the api such that it won't use the OS io scheduler, so the io comes directly to the block device driver through the 'make_request' call. With the device mapper i don't think that happens(looking at the api calls).
I believe that is correct and one of the reasons DRBD is not part of DM. DRBD has various modes including those where it guarantees the I/Os on both the main target and on the replicated target hit in the exact same order. I don't believe that DM can be used to totally control disk I/O. It is meant to be stackable, so I think you lose some exact control.
Does this mean that stuff like io scheduling. barrier control etc is done by the device mapper itself and we can focus only on 'mapping' the io.
As shown in the diagram linked above, DM sits above the i/o schedulers so you should not have to worry about it. If you want to play with schedulers, I think that should be done outside of DM. Thus I believe you can ignore barriers/scheduling UNLESS you create a target that needs special barrier/scheduling control. The obvious example of something needing that is raid5. If you get a barrier that forces data out to a single disk in a raid, you MUST ensure that the raid checksum is calculated and written out prior to calling the barrier complete. That is going to take special handling no matter what you do. It's been a couple years since I dug into raid 5/6 as relates to barriers, but it used to be that code simply didn't do the right thing in mdraid and DM did not support raid 5/6, so yes the coders could ignore it, but they created broken logic when they did. Greg
participants (6)
-
amit mehta -
Anatol Pomozov -
Gaurav Mahajan -
Greg Freemyer -
Kumar Amit Mehta
-
neha naik