On Sat, Sep 17, 2022 at 12:46:33PM -0700, Sarthak Kukreti wrote: > On Fri, Sep 16, 2022 at 8:03 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote: > > > > On Thu, Sep 15, 2022 at 09:48:18AM -0700, Sarthak Kukreti wrote: > > > From: Sarthak Kukreti <sarthakkukreti@xxxxxxxxxxxx> > > > > > > Hi, > > > > > > This patch series is an RFC of a mechanism to pass through provision > > > requests on stacked thinly provisioned storage devices/filesystems. > > > > [Reflowed text] > > > > > The linux kernel provides several mechanisms to set up thinly > > > provisioned block storage abstractions (eg. dm-thin, loop devices over > > > sparse files), either directly as block devices or backing storage for > > > filesystems. Currently, short of writing data to either the device or > > > filesystem, there is no way for users to pre-allocate space for use in > > > such storage setups. Consider the following use-cases: > > > > > > 1) Suspend-to-disk and resume from a dm-thin device: In order to > > > ensure that the underlying thinpool metadata is not modified during > > > the suspend mechanism, the dm-thin device needs to be fully > > > provisioned. > > > 2) If a filesystem uses a loop device over a sparse file, fallocate() > > > on the filesystem will allocate blocks for files but the underlying > > > sparse file will remain intact. > > > 3) Another example is virtual machine using a sparse file/dm-thin as a > > > storage device; by default, allocations within the VM boundaries will > > > not affect the host. > > > 4) Several storage standards support mechanisms for thin provisioning > > > on real hardware devices. For example: > > > a. The NVMe spec 1.0b section 2.1.1 loosely talks about thin > > > provisioning: "When the THINP bit in the NSFEAT field of the > > > Identify Namespace data structure is set to ‘1’, the controller ... > > > shall track the number of allocated blocks in the Namespace > > > Utilization field" > > > b. The SCSi Block Commands reference - 4 section references "Thin > > > provisioned logical units", > > > c. UFS 3.0 spec section 13.3.3 references "Thin provisioning". > > > > > > In all of the above situations, currently the only way for > > > pre-allocating space is to issue writes (or use > > > WRITE_ZEROES/WRITE_SAME). However, that does not scale well with > > > larger pre-allocation sizes. > > > > > > This patchset introduces primitives to support block-level > > > provisioning (note: the term 'provisioning' is used to prevent > > > overloading the term 'allocations/pre-allocations') requests across > > > filesystems and block devices. This allows fallocate() and file > > > creation requests to reserve space across stacked layers of block > > > devices and filesystems. Currently, the patchset covers a prototype on > > > the device-mapper targets, loop device and ext4, but the same > > > mechanism can be extended to other filesystems/block devices as well > > > as extended for use with devices in 4 a-c. > > > > If you call REQ_OP_PROVISION on an unmapped LBA range of a block device > > and then try to read the provisioned blocks, what do you get? Zeroes? > > Random stale disk contents? > > > > I think I saw elsewhere in the thread that any mapped LBAs within the > > provisioning range are left alone (i.e. not zeroed) so I'll proceed on > > that basis. > > > For block devices, I'd say it's definitely possible to get stale data, depending > on the implementation of the allocation layer; for example, with dm-thinpool, > the default setting via using LVM2 tools is to zero out blocks on allocation. > But that's configurable and can be turned off to improve performance. > > Similarly, for actual devices that end up supporting thin provisioning, unless > the specification absolutely mandates that an LBA contains zeroes post > allocation, some implementations will definitely miss out on that (probably > similar to the semantics of discard_zeroes_data today). I'm operating under > the assumption that it's possible to get stale data from LBAs allocated using > provision requests at the block layer and trying to see if we can create a > safe default operating model from that. Please explain the semantics of REQ_OP_PROVISION in the code/documentation in the next revision. Thanks, Stefan
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