EROFS has supported "native sub-filesystem merging" as a single block
device since kernel commit 8b465fecc35a ("erofs: support flattened block
device for multi-blob images"). It allows sub-filesystems (e.g., EROFS
container layers) to be merged into one filesystem and mounted in
a single shot, which is particularly useful for layered container images
in VM-based secure containers where file-backed mounts are unusable.
Additionally, it can also be used to pass through external data (such as
a tar file) w/o attaching an extra block device to the guest.
Intuitively, there are two approaches to generate a single merged block
device for virtualization scenarios:
- Concatenate fsmeta + sub-image files, for example, to merge
`[tar index][tar data]`:
$ mkfs.erofs --tar=i foo.erofs foo.tar
$ cat foo.tar >> foo.erofs
This approach is inefficient unless the host filesystem supports
reflinks: but a major player, EXT4, does not.
- Use the virtual block device approach on the host (e.g., NBD, TCMU,
UBLK) or vhost-user-blk to generate merged devices in the guest.
However, this requires an additional daemon to stay active, which can
be inconvenient.
Furthermore, I wondered whether any virtual disk format supports this
functionality. After doing some research on popular formats, I found
that only VMDK [1] and VHD [2] natively support merging split files.
QEMU appears to have supported VMDK split files [3] since very early
versions.
Add a `--vmdk-desc` option to generate valid `twoGbMaxExtentFlat` VMDK
descriptor files and use the following QEMU option to attach:
-drive file=foo.vmdk,format=vmdk,if=virtio
Hopefully, Cloud Hypervisor and other microVMs could support VMDK or
concatenating raw files as a single block device in the future.
[1] https://www.vmware.com/app/vmdk/?src=vmdk
[2] See `Splitting Hard Disk Images` in the VHD Format Specification
https://www.microsoft.com/en-us/download/details.aspx?id=23850
[3] https://github.com/qemu/qemu/blob/master/block/vmdk.c
Signed-off-by: Gao Xiang <hsiang...@linux.alibaba.com>
---
include/erofs/internal.h | 3 ++
lib/Makefile.am | 3 +-
lib/vmdk.c | 74 ++++++++++++++++++++++++++++++++++++++++
man/mkfs.erofs.1 | 4 +++
mkfs/main.c | 15 ++++++++
5 files changed, 98 insertions(+), 1 deletion(-)
create mode 100644 lib/vmdk.c
diff --git a/include/erofs/internal.h b/include/erofs/internal.h
index 8916be1..d3debc6 100644
--- a/include/erofs/internal.h
+++ b/include/erofs/internal.h
@@ -511,6 +511,9 @@ static inline int erofs_blk_read(struct erofs_sb_info *sbi,
int device_id,
erofs_pos(sbi, nblocks));
}
+/* vmdk.c */
+int erofs_dump_vmdk_desc(FILE *f, struct erofs_sb_info *sbi);
+
#ifdef EUCLEAN
#define EFSCORRUPTED EUCLEAN /* Filesystem is corrupted */
#else
diff --git a/lib/Makefile.am b/lib/Makefile.am
index bdc74ad..688403b 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -35,7 +35,8 @@ liberofs_la_SOURCES = config.c io.c cache.c super.c inode.c
xattr.c exclude.c \
namei.c data.c compress.c compressor.c zmap.c
decompress.c \
compress_hints.c hashmap.c sha256.c blobchunk.c dir.c \
fragments.c dedupe.c uuid_unparse.c uuid.c tar.c \
- block_list.c rebuild.c diskbuf.c bitops.c dedupe_ext.c
+ block_list.c rebuild.c diskbuf.c bitops.c dedupe_ext.c \
+ vmdk.c
liberofs_la_CFLAGS = -Wall ${libuuid_CFLAGS} -I$(top_srcdir)/include
if ENABLE_LZ4
diff --git a/lib/vmdk.c b/lib/vmdk.c
new file mode 100644
index 0000000..06d4a49
--- /dev/null
+++ b/lib/vmdk.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0+ OR Apache-2.0
+#include "erofs/internal.h"
+
+static int erofs_vmdk_desc_add_extent(FILE *f, u64 sectors,
+ const char *filename, u64 offset)
+{
+ static const char extent_line_fmt[] =
+ "RW %" PRIu64 " FLAT \"%s\" %" PRIu64 "\n";
+
+ while (sectors) {
+ u64 count = min_t(u64, sectors, 0x80000000 >> 9);
+ int ret;
+
+ ret = fprintf(f, extent_line_fmt, count, filename, offset);
+ if (ret < 0)
+ return -errno;
+ offset += count;
+ sectors -= count;
+ }
+ return 0;
+}
+
+int erofs_dump_vmdk_desc(FILE *f, struct erofs_sb_info *sbi)
+{
+ static const char desc_template_1[] =
+ "# Disk DescriptorFile\n"
+ "version=1\n"
+ "CID=%" PRIx32 "\n"
+ "parentCID=%" PRIx32 "\n"
+ "createType=\"%s\"\n"
+ "\n"
+ "# Extent description\n";
+ static const char desc_template_2[] =
+ "\n"
+ "# The Disk Data Base\n"
+ "#DDB\n"
+ "\n"
+ "ddb.virtualHWVersion = \"%s\"\n"
+ "ddb.geometry.cylinders = \"%" PRIu64 "\"\n"
+ "ddb.geometry.heads = \"%" PRIu32 "\"\n"
+ "ddb.geometry.sectors = \"63\"\n"
+ "ddb.adapterType = \"%s\"\n";
+ static const char subformat[] = "twoGbMaxExtentFlat";
+ static const char adapter_type[] = "ide";
+ u32 cid = ((u32 *)sbi->uuid)[0] ^ ((u32 *)sbi->uuid)[1] ^
+ ((u32 *)sbi->uuid)[2] ^ ((u32 *)sbi->uuid)[3];
+ u32 parent_cid = 0xffffffff;
+ u32 number_heads = 16;
+ char *hw_version = "4";
+ u64 total_sectors, sectors;
+ int ret, i;
+
+ fprintf(f, desc_template_1, cid, parent_cid, subformat);
+ sectors = sbi->primarydevice_blocks << (sbi->blkszbits - 9);
+ ret = erofs_vmdk_desc_add_extent(f, sectors, (char *)sbi->devname, 0);
+ if (ret)
+ return ret;
+ total_sectors = sectors;
+ for (i = 0; i < sbi->extra_devices; ++i) {
+ const char *name = sbi->devs[i].src_path ?:
+ (const char *)sbi->devs[i].tag;
+
+ sectors = sbi->devs[i].blocks << (sbi->blkszbits - 9);
+ ret = erofs_vmdk_desc_add_extent(f, sectors, name, 0);
+ if (ret)
+ return ret;
+ total_sectors += sectors;
+ }
+
+ fprintf(f, desc_template_2, hw_version,
+ (u64)DIV_ROUND_UP(total_sectors, 63ULL * number_heads),
+ number_heads, adapter_type);
+ return 0;
+}
diff --git a/man/mkfs.erofs.1 b/man/mkfs.erofs.1
index 48202b6..63f7a2f 100644
--- a/man/mkfs.erofs.1
+++ b/man/mkfs.erofs.1
@@ -270,6 +270,10 @@ together.
Filter tarball streams through xz, lzma, or lzip. Optionally, raw streams can
be dumped together.
.TP
+.BI "\-\-vmdk-desc=" FILE
+Generate a VMDK descriptor file to merge sub-filesystems, which can be used
+for tar index or rebuild mode.
+.TP
.BI "\-\-xattr-prefix=" PREFIX
Specify a customized extended attribute namespace prefix for space saving,
e.g. "trusted.overlay.". You may give multiple
diff --git a/mkfs/main.c b/mkfs/main.c
index ef83f2e..14ea6ff 100644
--- a/mkfs/main.c
+++ b/mkfs/main.c
@@ -90,6 +90,7 @@ static struct option long_options[] = {
{"async-queue-limit", required_argument, NULL, 530},
#endif
{"fsalignblks", required_argument, NULL, 531},
+ {"vmdk-desc", required_argument, NULL, 532},
{0, 0, 0, 0},
};
@@ -210,6 +211,7 @@ static void usage(int argc, char **argv)
" --unxz[=X] try to filter the tarball stream
through xz/lzma/lzip\n"
" (and optionally dump the raw stream to
X together)\n"
#endif
+ " --vmdk-desc=X generate a VMDK descriptor file to
merge sub-filesystems\n"
#ifdef EROFS_MT_ENABLED
" --workers=# set the number of worker threads to #
(default: %u)\n"
#endif
@@ -254,6 +256,7 @@ static bool valid_fixeduuid;
static unsigned int dsunit;
static unsigned int fsalignblks = 1;
static int tarerofs_decoder;
+static FILE *vmdk_dcf;
static int erofs_mkfs_feat_set_legacy_compress(bool en, const char *val,
unsigned int vallen)
@@ -988,6 +991,13 @@ static int mkfs_parse_options_cfg(int argc, char *argv[])
return -EINVAL;
}
break;
+ case 532:
+ vmdk_dcf = fopen(optarg, "wb");
+ if (!vmdk_dcf) {
+ erofs_err("failed to open vmdk desc `%s`",
optarg);
+ return -EINVAL;
+ }
+ break;
case 'V':
version();
exit(0);
@@ -1548,6 +1558,11 @@ int main(int argc, char **argv)
if (!err)
erofs_info("superblock checksum 0x%08x written", crc);
}
+
+ if (!err && vmdk_dcf) {
+ err = erofs_dump_vmdk_desc(vmdk_dcf, &g_sbi);
+ fclose(vmdk_dcf);
+ }
exit:
if (root)
erofs_iput(root);
--
2.43.5
