On 2/2/23 10:46, James Bottomley wrote:
Well, that may not be true in all situations. Semantically FUA is a
barrier: it can be implemented such that it destages only the current
write plus the cache writes that occurred before the write with the
FUA. It could also be implemented as you suggest above, which simply
destages the entire cache, but it doesn't have to be. One of the
reasons for FUA to exist is the potential difference between the two.
Hi James,
Although support for the barrier concept has been removed from the block
layer, would it be possible to tell me in which T10 document I can find
more information about the barrier semantics? All I found in the latest
SBC-5 draft (revision 4; 2023-01-24) about FUA is the following (section
5.40 WRITE (10)):
"A force unit access (FUA) bit set to one specifies that the device
server shall write the logical blocks to:
a) the non-volatile cache, if any; or
b) the medium.
An FUA bit set to zero specifies that the device server shall write the
logical blocks to:
a) volatile cache, if any;
b) non-volatile cache, if any; or
c) the medium."
To me the description of FUA in the SBC-3 draft from 11 November 2013
seems identical to the above text.
Thanks,
Bart.
