md: new bitmap sysfs interface

This patch (tested against 2.6.18-rc1-mm1) adds a new sysfs interface that allows the bitmap of an array to be dirtied. The interface is write-only, and is used as follows:

echo "1000" > /sys/block/md2/md/bitmap

(dirty the bit for chunk 1000 [offset 0] in the in-memory and on-disk bitmaps of array md2)

echo "1000-2000" > /sys/block/md1/md/bitmap

(dirty the bits for chunks 1000-2000 in md1's bitmap)

This is useful, for example, in cluster environments where you may need to combine two disjoint bitmaps into one (following a server failure, after a secondary server has taken over the array). By combining the bitmaps on the two servers, a full resync can be avoided (This was discussed on the list back on March 18, 2005, "[PATCH 1/2] md bitmap bug fixes" thread).

Thanks,
Paul

diff -pur linux-2.6.18-rc1-mm1/drivers/md/bitmap.c linux-2.6.18-rc1-mm1-bitmap-sysfs/drivers/md/bitmap.c
@@ -613,6 +613,7 @@ static inline unsigned long file_page_of
 static inline struct page *filemap_get_page(struct bitmap *bitmap,
 					unsigned long chunk)
 {
+	if (file_page_index(chunk) >= bitmap->file_pages) return NULL;
 	return bitmap->filemap[file_page_index(chunk) - file_page_index(0)];
 }
 

@@ -739,6 +740,7 @@ static void bitmap_file_set_bit(struct b
 	}
 
 	page = filemap_get_page(bitmap, chunk);
+	if (!page) return;
 	bit = file_page_offset(chunk);
 
  	/* set the bit */

@@ -1322,6 +1324,18 @@ static void bitmap_set_memory_bits(struc
 
 }
 
+/* dirty the memory and file bits for bitmap chunks "s" to "e" */
+void bitmap_dirty_bits(struct bitmap *bitmap, unsigned long s, unsigned long e)
+{
+	unsigned long chunk;
+
+	for (chunk = s; chunk <= e; chunk++) {
+		sector_t sec = chunk << CHUNK_BLOCK_SHIFT(bitmap);
+		bitmap_set_memory_bits(bitmap, sec, 1);
+		bitmap_file_set_bit(bitmap, sec);
+	}
+}
+
 /*
  * flush out any pending updates
  */

diff -pur linux-2.6.18-rc1-mm1/drivers/md/md.c linux-2.6.18-rc1-mm1-bitmap-sysfs/drivers/md/md.c
@@ -2507,6 +2507,36 @@ static struct md_sysfs_entry md_new_devi
 __ATTR(new_dev, S_IWUSR, null_show, new_dev_store);
 
 static ssize_t
+bitmap_store(mddev_t *mddev, const char *buf, size_t len)
+{
+	char *end;
+	unsigned long chunk, end_chunk;
+
+	if (!mddev->bitmap)
+		goto out;
+	/* buf should be <chunk> <chunk> ... or <chunk>-<chunk> ... (range) */
+	while (*buf) {
+		chunk = end_chunk = simple_strtoul(buf, &end, 0);
+		if (buf == end) break;
+		if (*end == '-') { /* range */
+			buf = end + 1;
+			end_chunk = simple_strtoul(buf, &end, 0);
+			if (buf == end) break;
+		}
+		if (*end && !isspace(*end)) break;
+		bitmap_dirty_bits(mddev->bitmap, chunk, end_chunk);
+		buf = end;
+		while (isspace(*buf)) buf++;
+	}
+	bitmap_unplug(mddev->bitmap); /* flush the bits to disk */
+out:
+	return len;
+}
+
+static struct md_sysfs_entry md_bitmap =
+__ATTR(bitmap, S_IWUSR, null_show, bitmap_store);
+
+static ssize_t
 size_show(mddev_t *mddev, char *page)
 {
 	return sprintf(page, "%llu\n", (unsigned long long)mddev->size);

@@ -2826,6 +2856,7 @@ static struct attribute *md_redundancy_a
 	&md_sync_completed.attr,
 	&md_suspend_lo.attr,
 	&md_suspend_hi.attr,
+	&md_bitmap.attr,
 	NULL,
 };
 static struct attribute_group md_redundancy_group = {

Hi Paul,

I tracked down the thread you referenced and these posts (by you) seems to summarize things well:
http://marc.theaimsgroup.com/?l=linux-raid&m=111116563016418&w=2
http://marc.theaimsgroup.com/?l=linux-raid&m=111117515400864&w=2

But for clarity's sake, could you elaborate on the negative implications of not merging the bitmaps on the secondary server? Will the previous primary's dirty blocks get dropped on the floor because the secondary (now the primary) doesn't have awareness of the previous primary's dirty blocks once it activates the raid1?

Also, what is the interface one should use to collect dirty bits from the primary's bitmap?

This bitmap merge can't happen until the primary's dirty bits can be collected right? Waiting for the failed server to come back to harvest the dirty bits it has seems wrong (why failover at all?); so I must be missing something.

please advise, thanks.
Mike

Right. At the time of the failover, there were (probably) blocks that were out of sync between the primary and secondary. Now, after you've failed over to the secondary, you've got to overwrite those blocks with data from the secondary in order to make the primary disk consistent again. This requires that either you do a full resync from secondary to primary (if you don't know what differs), or you merge the two bitmaps and resync just that data.

Whatever you'd like. scp the bitmap file over or collect the ranges into a file and scp that over, or something similar.

Right. So, when the primary fails, you start the array on the secondary with a _clean_ bitmap, and just its local disk component. Now, whatever gets written while the primary is down gets put into the bitmap on the secondary. When the primary comes back up, you take the dirty bits from it and add them into the secondary's bitmap. Then, you insert the primary's disk (via nbd or similar) back into the array, and begin a resync.

That's the whole reason for this interface. We have to modify the bitmap while the array is active (modifying the bitmap while the array is down is trivial, and certainly doesn't require sysfs :).

We fail over immediately. We wait until later to combine the bitmaps and resync the data.

Hope that helps.

--
Paul

I took more time to read the later posts in the original thread; that coupled with your detailed response has helped a lot. thanks.

OK, so regardless of whether you are using an external or internal bitmap; how does one collect the ranges from an array's bitmap?

Generally speaking I think others would have the same (naive) question given that we need to know what to use as input for the sysfs
interface you've kindly provided. If it is left as an exercise to
the user that is fine; I'd imagine neilb will get our backs with a nifty new mdadm flag if need be.

thanks again,
Mike

Well, with an internal bitmap you don't need this interface. The bitmap is located on all the component disks. The reason we don't use internal bitmaps in a configuration where one of the disks is located remotely (over a LAN or SAN, or possibly a WAN) is that the bitmap updates (which are synchronous and occur fairly often) would be too costly.

So reading the bits out of a file is fairly simple. The bitmap file is laid out one bit per chunk, with a 256 byte superblock at the front. You just need a perl script (for example) that reads the file and keeps track of which bits are dirty, and then prints those numbers out.

--
Paul

OK, so now that I understand the need to merge the bitmaps... the various scenarios that create this (potential) inconsistency are still unclear to me when you consider the different flavors of raid1. Is this inconsistency only possible if using async (aka write-behind) raid1?

If not, how would this difference in committed blocks occur with normal (sync) raid1 given MD's endio acknowledges writes after they are submitted to all raid members? Is it merely that the bitmap is left with dangling bits set that don't reflect reality (blocks weren't actually changed anywhere) when a crash occurs? Is there real potential for inconsistent data on disk(s) when using sync raid1 (does having an nbd member increase the likelihood)?

regards,
Mike

No. Even with a synchronous (normal) raid1, you will probably have blocks that are out of sync when one disk (or server) fails. This is true even of raid1's using internal disks. That's why you resync the array after a failure (of the system or of one of the disks). That's exactly what the bitmap is for -- to optimize that resync.

--
Paul

Thanks. Only one remaining issue (which I didn't think of last time :-)

Is 'bitmap' the best name for the sysfs file?
It seems a bit generic to me.

write-bits-here-to-dirty-them-in-the-bitmap

is probably (no, definitely) too verbose.

dirty-in-bitmap
maybe?
bitmap-set-bits

None of these seem completely satisfying.
I like to start with the most important word - which is 'bitmap' in this case, but that isn't completely essential.

Any better suggestions?

Thanks,
NeilBrown

I like "bitmap-set-bits" or "bitmap-dirty" maybe. I had thought about this, but never changed the name. I agree, something more than just "bitmap" is probably appropriate, so that if we ever add a generic bitmap output or input (to clear bits, maybe) then we still have "bitmap" available, and also because (as you said) "bitmap" is a little too generic for what this interface really does.

Thanks,
Paul

It's not actually a bitmap is it?
It takes a number or range and *operates* on a bitmap.

so:
dirty-chunk-in-bitmap

or maybe:
dirty-bitmap-chunk

David

Hello All ,

A thought , maybe bitmap-dirty-this-chunk ?

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