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https://communities.vmware.com/thread/65305
Feb 24, 2010 · Furthermore regarding your comments about Forced Unit Access and write cache, FUA is a bit that is enabled or disabled during the write operation. When this is enabled the write is not returned as successful until it is physically written to disk.
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http://lkml.iu.edu/hypermail/linux/kernel/1209.0/00812.html
and REQ_FUA bits get stripped away if the queue does not advertise REQ_FLUSH or REQ_FUA support. But the REQ_FLUSH bit is also tested for when not merging requests (blk_queue_bio()) or when it comes to the elevator (blk_flush_plug_list()). So, since this patch the elevator reorders write requests on queues that do not have REQ_FLUSH or REQ_FUA set.
https://stackoverflow.com/questions/41602619/fua-flush-and-ordering
Jan 12, 2017 · A FUA command usually means the data needs to be committed to the NVM before signalling completion. ... FUA, flush and ordering. ... Write X with FUA bit set Alternatively, if the HW doesn't support FUA, this can be emulated by Write X;
https://www.kernel.org/doc/html/latest/block/writeback_cache_control.html
For devices that also support the FUA bit the block layer needs to be told to pass through the REQ_FUA bit using: blk_queue_write_cache(sdkp->disk->queue, true, true); and the driver must handle write requests that have the REQ_FUA bit set in prep_fn/request_fn. If the FUA bit is not natively supported the block layer turns it into an empty REQ ...
http://sourceforge.net/p/scst/mailman/message/28209992/
When write_through is enabled, all writes are handled as if they had the FUA bit set. Having write_through support is useful to disable all disk caching on the target, thus preventing the "iSCSI target reboot write hole" issue. More details are available in the patch headers. These patches are against the SCST trunk r3843.
http://www.joshodgers.com/tag/forced-unit-access-fua-write-through/
Nov 10, 2014 · Forced Unit Access (FUA) & Write-Through is supported by VMware but even with this support, it is also a function of the underlying storage to honour the request and this process or even support may vary from storage vendor to storage vendor.
https://docs.microsoft.com/en-us/windows-hardware/drivers/storage/querying-for-the-write-cache-property
If the device supports write-through requests, the initiator can bypass the write cache by setting the force unit access (FUA) bit in the command descriptor block (CDB) of the write command. Sync cache support. The write cache property indicates whether the device supports the SCSI SYNC CACHE command, or an equivalent command on other buses.
https://serverfault.com/questions/229443/what-is-read-cache-in-sda-write-cache-enabled-read-cache-enabled-doesnt
I'm trying to figure out what the read cache part of the following message refers to: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA. Is it "IDE drive's read-lookahead feature (usually ON by default)" as treated by hdparm?
https://blogs.technet.microsoft.com/storageserver/2013/09/27/windows-storage-server-2012-r2-whats-new-for-smb-and-the-iscsi-target-server/
Sep 27, 2013 · Optimizations to enable disk-level caching, which can improve performance when you have Force Unit Access (FUA)-capable back-end disks or JBODs behind the iSCSI Target Server. The FUA bit tells the target to immediately send the data to the …
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