diff options
| -rw-r--r-- | daemon/zero.c | 81 | ||||
| -rw-r--r-- | generator/generator_actions.ml | 10 |
2 files changed, 70 insertions, 21 deletions
diff --git a/daemon/zero.c b/daemon/zero.c index c8e79ae9..c9f6bf7f 100644 --- a/daemon/zero.c +++ b/daemon/zero.c @@ -28,26 +28,57 @@ #include "daemon.h" #include "actions.h" +/* Return true iff the buffer is all zero bytes. + * + * Note that gcc is smart enough to optimize this properly: + * http://stackoverflow.com/questions/1493936/faster-means-of-checking-for-an-empty-buffer-in-c/1493989#1493989 + */ +static int +is_zero (const char *buffer, size_t size) +{ + size_t i; + + for (i = 0; i < size; ++i) { + if (buffer[i] != 0) + return 0; + } + + return 1; +} + +static const char zero_buf[4096]; + int do_zero (const char *device) { - int fd, i; - char buf[4096]; + char buf[sizeof zero_buf]; + int fd; + size_t i, offset; - fd = open (device, O_WRONLY); + fd = open (device, O_RDWR); if (fd == -1) { reply_with_perror ("%s", device); return -1; } - memset (buf, 0, sizeof buf); - for (i = 0; i < 32; ++i) { - if (write (fd, buf, sizeof buf) != sizeof buf) { - reply_with_perror ("write: %s", device); + offset = i * sizeof zero_buf; + + /* Check if the block is already zero before overwriting it. */ + if (pread (fd, buf, sizeof buf, offset) != sizeof buf) { + reply_with_perror ("pread: %s", device); close (fd); return -1; } + + if (!is_zero (buf, sizeof buf)) { + if (pwrite (fd, zero_buf, sizeof zero_buf, offset) != sizeof zero_buf) { + reply_with_perror ("pwrite: %s", device); + close (fd); + return -1; + } + } + notify_progress ((uint64_t) i, 32); } @@ -67,14 +98,13 @@ do_zero_device (const char *device) return -1; uint64_t size = (uint64_t) ssize; - int fd = open (device, O_WRONLY); + int fd = open (device, O_RDWR); if (fd == -1) { reply_with_perror ("%s", device); return -1; } - char buf[1024*1024]; - memset (buf, 0, sizeof buf); + char buf[sizeof zero_buf]; uint64_t pos = 0; @@ -86,15 +116,28 @@ do_zero_device (const char *device) else n = (size_t) n64; /* safe because of if condition */ - ssize_t r = write (fd, buf, n); + /* Check if the block is already zero before overwriting it. */ + ssize_t r; + r = pread (fd, buf, n, pos); if (r == -1) { - reply_with_perror ("write: %s (with %" PRId64 " bytes left to write)", - device, size); + reply_with_perror ("pread: %s at offset %" PRIu64, device, pos); close (fd); return -1; } - pos += r; + if (!is_zero (buf, sizeof buf)) { + r = pwrite (fd, zero_buf, n, pos); + if (r == -1) { + reply_with_perror ("pwrite: %s (with %" PRId64 " bytes left to write)", + device, size); + close (fd); + return -1; + } + pos += r; + } + else + pos += n; + notify_progress (pos, size); } @@ -106,13 +149,11 @@ do_zero_device (const char *device) return 0; } -static char zero[BUFSIZ]; - int do_is_zero (const char *path) { int fd; - char buf[BUFSIZ]; + char buf[1024*1024]; ssize_t r; CHROOT_IN; @@ -125,7 +166,7 @@ do_is_zero (const char *path) } while ((r = read (fd, buf, sizeof buf)) > 0) { - if (memcmp (buf, zero, r) != 0) { + if (!is_zero (buf, r)) { close (fd); return 0; } @@ -149,7 +190,7 @@ int do_is_zero_device (const char *device) { int fd; - char buf[BUFSIZ]; + char buf[1024*1024]; ssize_t r; fd = open (device, O_RDONLY); @@ -159,7 +200,7 @@ do_is_zero_device (const char *device) } while ((r = read (fd, buf, sizeof buf)) > 0) { - if (memcmp (buf, zero, r) != 0) { + if (!is_zero (buf, r)) { close (fd); return 0; } diff --git a/generator/generator_actions.ml b/generator/generator_actions.ml index c684e358..a924d87a 100644 --- a/generator/generator_actions.ml +++ b/generator/generator_actions.ml @@ -3006,6 +3006,10 @@ How many blocks are zeroed isn't specified (but it's I<not> enough to securely wipe the device). It should be sufficient to remove any partition tables, filesystem superblocks and so on. +If blocks are already zero, then this command avoids writing +zeroes. This prevents the underlying device from becoming non-sparse +or growing unnecessarily. + See also: C<guestfs_zero_device>, C<guestfs_scrub_device>, C<guestfs_is_zero_device>"); @@ -5229,7 +5233,11 @@ is not large enough."); "\ This command writes zeroes over the entire C<device>. Compare with C<guestfs_zero> which just zeroes the first few blocks of -a device."); +a device. + +If blocks are already zero, then this command avoids writing +zeroes. This prevents the underlying device from becoming non-sparse +or growing unnecessarily."); ("txz_in", (RErr, [FileIn "tarball"; Pathname "directory"], []), 229, [Optional "xz"], [InitScratchFS, Always, TestOutput ( |