Documentation: Update block cache mode information

file sectors into the image file.

@end table

By default, writethrough caching is used for all block device.  This means that

the host page cache will be used to read and write data but write notification

will be sent to the guest only when the data has been reported as written by

the storage subsystem.

Writeback caching will report data writes as completed as soon as the data is

present in the host page cache.  This is safe as long as you trust your host.

If your host crashes or loses power, then the guest may experience data

corruption.

By default, the @option{cache=writeback} mode is used. It will report data

writes as completed as soon as the data is present in the host page cache.

This is safe as long as your guest OS makes sure to correctly flush disk caches

where needed. If your guest OS does not handle volatile disk write caches

correctly and your host crashes or loses power, then the guest may experience

data corruption.

For such guests, you should consider using @option{cache=writethrough}. This

means that the host page cache will be used to read and write data, but write

notification will be sent to the guest only after QEMU has made sure to flush

each write to the disk. Be aware that this has a major impact on performance.

The host page cache can be avoided entirely with @option{cache=none}.  This will

attempt to do disk IO directly to the guests memory.  QEMU may still perform

an internal copy of the data.

attempt to do disk IO directly to the guest's memory.  QEMU may still perform

an internal copy of the data. Note that this is considered a writeback mode and

the guest OS must handle the disk write cache correctly in order to avoid data

corruption on host crashes.

The host page cache can be avoided while only sending write notifications to

the guest when the data has been reported as written by the storage subsystem

using @option{cache=directsync}.

Some block drivers perform badly with @option{cache=writethrough}, most notably,

qcow2.  If performance is more important than correctness,

@option{cache=writeback} should be used with qcow2.

the guest when the data has been flushed to the disk using

@option{cache=directsync}.

In case you don't care about data integrity over host failures, use

cache=unsafe. This option tells QEMU that it never needs to write any data

to the disk but can instead keeps things in cache. If anything goes wrong,

@option{cache=unsafe}. This option tells QEMU that it never needs to write any

data to the disk but can instead keep things in cache. If anything goes wrong,

like your host losing power, the disk storage getting disconnected accidentally,

etc. you're image will most probably be rendered unusable.   When using

etc. your image will most probably be rendered unusable.   When using

the @option{-snapshot} option, unsafe caching is always used.

Copy-on-read avoids accessing the same backing file sectors repeatedly and is