block: advisory locks: use byte-range locks to match QEMU behavior #7494
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The granularity has significant implications in typical cloud deployments with network storage. The Linux kernel will sync advisory locks to network file systems, but these backends may have different policies and handle locks differently. For example, Netapp speaks a NFS API but will treat advisory OFD locks for the whole file as mandatory locks, whereas byte-range locks for the whole file will remain advisory [0]. As it is a valid use case to prevent multiple CHV instances from accessing the same disk but disk management software (e.g., Cinder in OpenStack) should be able to snapshot disks while VMs are running, we need special control over the lock granularity. Therefore, it is a valid use case to lock the whole byte range of a disk image without technically locking the whole file - to get the best of both worlds. This also brings CHVs behavior in line with QEMU [1]. Whole-file locks remain a valid use case and could be supported later. This patch only provides the necessary groundwork; making it configurable is out of scope for now. [0] https://kb.netapp.com/on-prem/ontap/da/NAS/NAS-KBs/How_is_Mandatory_Locking_supported_for_NFSv4_on_ONTAP_9 [1] <qemu>/util/osdep.c::qemu_lock_fcntl() Signed-off-by: Philipp Schuster <[email protected]> On-behalf-of: SAP [email protected]
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Signed-off-by: Philipp Schuster <[email protected]> On-behalf-of: SAP [email protected]
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Follow-up of #6974. Please see the commit message for more context.
Our customer/partner confirmed that this solves the problems with NetApp and this is also the same thing that QEMU does. My implementation leaves room for adding
--disk lock_granularity=full|byte-rangein the future, if one ever needs that.PS: For now, we at least have testing in our internal test suite for this.