block: add -drive copy-on-read=on|off

[mirror_qemu.git] / qemu-options.hx

diff --git a/qemu-options.hx b/qemu-options.hx
index 25a7be7948d034ce60e4bcce45e8712c658f858b..b3db10c0ad196b4cdf3d6c10d7c2274170cc0add 100644 (file)
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -135,7 +135,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n"
     "       [,cache=writethrough|writeback|none|directsync|unsafe][,format=f]\n"
     "       [,serial=s][,addr=A][,id=name][,aio=threads|native]\n"
-    "       [,readonly=on|off]\n"
+    "       [,readonly=on|off][,copy-on-read=on|off]\n"
     "       [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]][[,iops=i]|[[,iops_rd=r][,iops_wr=w]]\n"
     "                use 'file' as a drive image\n", QEMU_ARCH_ALL)
 STEXI
@@ -187,6 +187,9 @@ host disk is full; report the error to the guest otherwise).
 The default setting is @option{werror=enospc} and @option{rerror=report}.
 @item readonly
 Open drive @option{file} as read-only. Guest write attempts will fail.
+@item copy-on-read=@var{copy-on-read}
+@var{copy-on-read} is "on" or "off" and enables whether to copy read backing
+file sectors into the image file.
 @end table
 
 By default, writethrough caching is used for all block device.  This means that
@@ -218,6 +221,10 @@ like your host losing power, the disk storage getting disconnected accidently,
 etc. you're 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
+useful when the backing file is over a slow network.  By default copy-on-read
+is off.
+
 Instead of @option{-cdrom} you can use:
 @example
 qemu -drive file=file,index=2,media=cdrom