Hannes Reinecke wrote: > On 06/01/2014 10:46 PM, Sebastian Herbszt wrote: > > Hello, > > > > I am trying to find out the status of SCSI referrals support in dm-multipath. > > SCSI referrals support was a topic at LSF [1] but I wasn't able to find any > > information. Back in 2010 Hannes suggested the use of multiple entries in a > > multipath table [2] but this doesn't seem to work (anymore): > > > > [ 7750.354190] device-mapper: table: Request-based dm doesn't support multiple targets yet > > > > Is there any SCSI referrals support yet? > > > > [1] http://marc.info/?l=linux-scsi&m=129553880112460&w=2 > > [2] http://www.redhat.com/archives/dm-devel/2010-June/msg00155.html > > > Not as of now. I've managed to push referrals support into > target_core, so you now can set up a backend which supports referrals. I gathered the following information from a storage with referral support. > And for the multiple entries I've though to use a linear target on > top of the multipath target. > The linear target would just split up the device in chunks, > according to the referrals layout. I created a new thin provisioned volume which reported Referrals VPD page (SBC): User data segment size: 0 User data segment multiplier: 0 and didn't return any user data segment referral descriptors. Since the chunk size is not known, splitting the device is not possible. The storage reported 4 equal target port groups (sd[b-e]). I guess the initial setup would consist of a linear target with just a single mapping. As soon as I started to fill the volume the storage returned the descriptors Report referrals: descriptor 0: target port descriptors: 4 user data segment: first lba 0, last lba 2752511 target port descriptor 0: port group 8080 state (active/non optimized) target port descriptor 1: port group 8081 state (active/non optimized) target port descriptor 2: port group 8090 state (active/optimized) target port descriptor 3: port group 8091 state (active/optimized) descriptor 1: target port descriptors: 4 user data segment: first lba 2752512, last lba 8257535 target port descriptor 0: port group 8080 state (active/optimized) target port descriptor 1: port group 8081 state (active/optimized) target port descriptor 2: port group 8090 state (active/non optimized) target port descriptor 3: port group 8091 state (active/non optimized) descriptor 2: target port descriptors: 4 user data segment: first lba 8257536, last lba 11010047 target port descriptor 0: port group 8080 state (active/non optimized) target port descriptor 1: port group 8081 state (active/non optimized) target port descriptor 2: port group 8090 state (active/optimized) target port descriptor 3: port group 8091 state (active/optimized) ... The first descriptor covers 1344 MB; the second twice as much. On a full volume the smallest size was 1344MB and few descriptors covered multiple of it. After running a rebalance operation on the storage the layout partially changed Report referrals: descriptor 0: target port descriptors: 4 user data segment: first lba 0, last lba 2752511 target port descriptor 0: port group 8080 state (active/optimized) target port descriptor 1: port group 8081 state (active/optimized) target port descriptor 2: port group 8090 state (active/non optimized) target port descriptor 3: port group 8091 state (active/non optimized) descriptor 1: target port descriptors: 4 user data segment: first lba 2752512, last lba 5505023 target port descriptor 0: port group 8080 state (active/non optimized) target port descriptor 1: port group 8081 state (active/non optimized) target port descriptor 2: port group 8090 state (active/optimized) target port descriptor 3: port group 8091 state (active/optimized) descriptor 2: target port descriptors: 4 user data segment: first lba 5505024, last lba 8257535 target port descriptor 0: port group 8080 state (active/optimized) target port descriptor 1: port group 8081 state (active/optimized) target port descriptor 2: port group 8090 state (active/non optimized) target port descriptor 3: port group 8091 state (active/non optimized) descriptor 3: target port descriptors: 4 user data segment: first lba 8257536, last lba 11010047 target port descriptor 0: port group 8080 state (active/non optimized) target port descriptor 1: port group 8081 state (active/non optimized) target port descriptor 2: port group 8090 state (active/optimized) target port descriptor 3: port group 8091 state (active/optimized) ... Is there currently a way to get notified on such changes? If we start with a single mapping we need a way to trigger a (re-)discovery/modification. > And then you would have several multipath devices (all referring to > the same disk), one for each supported referrals ALUA configuration. > The linear target would then map each chunk to the correct multipath > device. I assume the above configuration would require two multipath devices; one for target port descriptor 0: port group 8080 state (active/optimized) target port descriptor 1: port group 8081 state (active/optimized) target port descriptor 2: port group 8090 state (active/non optimized) target port descriptor 3: port group 8091 state (active/non optimized) and a second for target port descriptor 0: port group 8080 state (active/non optimized) target port descriptor 1: port group 8081 state (active/non optimized) target port descriptor 2: port group 8090 state (active/optimized) target port descriptor 3: port group 8091 state (active/optimized) > It'll be a beast for failover, but should be possible. > And maybe it's better to use dm-switch here; the number or referrals > might overflow dm-linear ... Does dm-switch work with different chunk sizes? Is there a theoretical or practical limit for dm-linear? My 100 GB device consists of 62 descriptors. > Cheers, > > Hannes Sebastian -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
