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[Qemu-devel] [PATCH 2/3] block: Do not cache device size for removable m

From: Stefan Hajnoczi
Subject: [Qemu-devel] [PATCH 2/3] block: Do not cache device size for removable media
Date: Wed, 23 Mar 2011 17:40:54 +0000

The block layer caches the device size to avoid doing lseek(fd, 0,
SEEK_END) every time this value is needed.  For removable media the
device size becomes stale if a new medium is inserted.  This patch
simply prevents device size caching for removable media.

A smarter solution is to update the cached device size when a new medium
is inserted.  Given that there are currently bugs with CD-ROM media
change I do not want to implement that approach until we've gotten
things correct first.

Signed-off-by: Stefan Hajnoczi <address@hidden>
 block.c |   12 +++++-------
 1 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/block.c b/block.c
index 8f224b4..89f6ded 100644
--- a/block.c
+++ b/block.c
@@ -1153,14 +1153,12 @@ int64_t bdrv_getlength(BlockDriverState *bs)
     if (!drv)
         return -ENOMEDIUM;
-    /* Fixed size devices use the total_sectors value for speed instead of
-       issuing a length query (like lseek) on each call.  Also, legacy block
-       drivers don't provide a bdrv_getlength function and must use
-       total_sectors. */
-    if (!bs->growable || !drv->bdrv_getlength) {
-        return bs->total_sectors * BDRV_SECTOR_SIZE;
-    }
-    return drv->bdrv_getlength(bs);
+    if (bs->growable || bs->removable) {
+        if (drv->bdrv_getlength) {
+            return drv->bdrv_getlength(bs);
+        }
+    }
+    return bs->total_sectors * BDRV_SECTOR_SIZE;
 /* return 0 as number of sectors if no device present or error */

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