Hello, I'm looking at extending dm-log-writes to support multiple devices to log to a single log. The benefit for this is testing things like btrfs's raid code, the xfs realtime device thing, and even mdraid. I left room in the log format to change it as needed with this use case in mind, so I'm not worried about that. I'm more looking for verification that my plan doesn't suck, or if it does suck what would be a better approach. I'll lay out the different parts to try and make this as quick and concise as possible. 1) Add a "log-writes-log" target that just takes the single device we are going to use as the log. This will do the work of taking the log IO from the actual "log-writes" target and putting it in the log. 2) Extend the "log-writes" target table format to include an "id" at the end of the table line that will be used to indicate which device the log entry will be fore. In this case the "log device" portion will point at the "log-writes-log" device mapper target. Everything would work normally, except in this mode we send the log bio's down with bi_sector = 0 and let the "log-writes-log" target do the actual mapping for the bio. So to test with multiple devices you would have to do something like dmsetup create log --table "0 <size> log-writes-log <log device>" dmsetup create lw1 --table "0 <size> log-writes <device> /dev/mapper/log 0" dmsetup create lw2 --table "0 <size> log-writes <device> /dev/mapper/log 1" mkfs.btrfs -d raid1 -m raid1 /dev/mapper/lw1 /dev/mapper/lw2 mount /dev/mapper/lw1 /mnt <do whatever> umount /mnt dmsetup remove lw1 dmsetup remove lw2 dmsetup remove log Mike, I would simply add a new struct target_type for log-writes-log that would do it's own ->map function to re-route the bio's coming into them. I'd also change the ->ctr function of the log-writes target_type to handle the new id field and do the new fancy thing if we have that field populated. Does that sound reasonable from an implementation point of view? Any comments or suggestions are welcome. I haven't written any code yet so I'm open to other ideas. Thanks, Josef
