Hi.

On 05/08/2018 03:17 PM, Kent Overstreet wrote:

s/,/;/

+ * various btrees. There are only a small number of btrees, they're not
+ * sharded - we have one btree for extents, another for inodes, et cetera.

   or shared?

+ *
+ * SUPERBLOCK:
+ *
+ * The superblock contains the location of the journal, the list of devices in
+ * the filesystem, and in general any metadata we need in order to decide
+ * whether we can start a filesystem or prior to reading the journal/btree
+ * roots.

[snip]

+struct bkey_format {
+	__u8		key_u64s;
+	__u8		nr_fields;
+	/* One unused slot for now: */
+	__u8		bits_per_field[6];
+	__le64		field_offset[6];
+};
+
+/* Btree keys - all units are in sectors */

Are sectors fixed size?  I.e., can 2 different physical storage devices have
different sized sectors?
or is this just the "traditional" 512-byte sector?


[snip]

+/* Extents */
+
+/*
+ * In extent bkeys, the value is a list of pointers (bch_extent_ptr), optionally
+ * preceded by checksum/compression information (bch_extent_crc32 or
+ * bch_extent_crc64).
+ *
+ * One major determining factor in the format of extents is how we handle and
+ * represent extents that have been partially overwritten and thus trimmed:
+ *
+ * If an extent is not checksummed or compressed, when the extent is trimmed we
+ * don't have to remember the extent we originally allocated and wrote: we can
+ * merely adjust ptr->offset to point to the start of the start of the data that

                                         to the start of the start  [intentional?]

+ * is currently live. The size field in struct bkey records the current (live)
+ * size of the extent, and is also used to mean "size of region on disk that we
+ * point to" in this case.

[snip]

+/*
+ * @offset	- sector where this sb was written
+ * @version	- on disk format version
+ * @magic	- identifies as a bcachefs superblock (BCACHE_MAGIC)
+ * @seq		- incremented each time superblock is written
+ * @uuid	- used for generating various magic numbers and identifying
+ *                member devices, never changes
+ * @user_uuid	- user visible UUID, may be changed
+ * @label	- filesystem label
+ * @seq		- identifies most recent superblock, incremented each time
+ *		  superblock is written
+ * @features	- enabled incompatible features
+ */
+struct bch_sb {
+	struct bch_csum		csum;
+	__le64			version;
+	uuid_le			magic;
+	uuid_le			uuid;
+	uuid_le			user_uuid;
+	__u8			label[BCH_SB_LABEL_SIZE];
+	__le64			offset;
+	__le64			seq;
+
+	__le16			block_size;
+	__u8			dev_idx;
+	__u8			nr_devices;
+	__le32			u64s;
+
+	__le64			time_base_lo;
+	__le32			time_base_hi;
+	__le32			time_precision;
+
+	__le64			flags[8];
+	__le64			features[2];
+	__le64			compat[2];
+
+	struct bch_sb_layout	layout;
+
+	union {
+		struct bch_sb_field start[0];
+		__le64		_data[0];
+	};
+} __attribute__((packed, aligned(8)));

I know that you have already answered a few comments about endianness,
so maybe you answered this and I missed it.

Can a bcachefs fs be shared, a la NFS?  I.e., can multiple different-endian
clients be accessing the same bcachefs?

thanks,
-- 
~Randy