Hi Martin, I'm trying to solve this "squaring the circle" problem: - my distributed block device "distributes" the data with the configured block size (e.g. 64K) to different of the many underlying slave block devices (here: dm-cache). - my block device evaluates the discard for logically discarding the specified block (it doesn't have more than 64K in a row) for some specific (logical) slave device. - that's why, I have to consider splitting the discard myself (if the block layer doesn't do it) for treating each 64K chunk on different (logical) slave devices. - on the underlying block device (e.g. dm-cache) the data is placed also in 64K chunks (there is not more in row). - that's why, ignoring this size and sending the original big size to it will probably delete unrelated data. Now my experiments show that, at least, dm-cache doesn't complain nor rejects those smaller discards than its discard_granularity, but possibly turning them into noop (?). May be that the needed functionality of accumulating small discards to a big one covering its own granularity (similar to SSDs block erasure) should be done at that driver level. What do you think ? Florian On Fri, Jul 29, 2016 at 3:39 AM, Martin K. Petersen <martin.petersen@xxxxxxxxxx> wrote: >>>>>> "Florian-Ewald" == Florian-Ewald Müller <florian-ewald.mueller@xxxxxxxxxxxxxxxx> writes: > > Florian-Ewald, > >> My (new developed) distributed block device uses a block size of 64K. > >> So, because of the distribution functionality to different underlying >> slave (dm-cache) block devices, it has: >> - max_discard_sectors: 128 (64K) >> - discard_granularity: 65536 (64K) > >> The discard sizes of the (underlying) dm-cache devices are: >> - max_discard_sectors: 536870912 (256G) >> - discard_granularity: 268435456 (256M) > > Why do you set max_discard_sectors for your device to 128? To my > knowledge dm-cache does not do partial allocation. And consequently any > discard less than an aligned 256MB will be a nop anyway. > > -- > Martin K. Petersen Oracle Linux Engineering -- Florian-Ewald Mueller Architecture Board ProfitBricks GmbH Greifswalder Str. 207 D - 10405 Berlin Tel: +49 30 577 008 331 Fax: +49 30 577 008 598 Email: florian-ewald.mueller@xxxxxxxxxxxxxxxx URL: http://www.profitbricks.de Sitz der Gesellschaft: Berlin. Registergericht: Amtsgericht Charlottenburg, HRB 125506 B. Geschäftsführer: Andreas Gauger, Achim Weiss. Please consider the environment before printing this email. -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
