Commit 9fe15314 authored by Mike Hibler's avatar Mike Hibler

A few notes about needed data structure improvements before I forget...

parent 398364eb
......@@ -251,8 +251,20 @@ Immediate goals:
the image files modtime whenever it reads a chunk of data and pass that
in the packet header (or, just outright fail if that value changes).
(the following are strictly imagezip):
5. Support for ext4 and LVM. These are coming sooner, rather than later.
6. Revisit imagezip data structures. Just imaging my modest 1TB disk at
home (re-)revealed an issue. It took more than 30 minutes just to get
to the point of creating the image, because of our singlely-linked list
representation of free blocks and the bubble sort we use to sort it.
This was with only O(100,000) free block ranges. We need a better free
(or allocated) range representation. We also need to resize some fields,
in particular in the range descriptors and other structs that pass block
numbers of range sizes. These should be 64-bit blocks numbers and sizes.
See imagezip/TODO for more thoughts.
Slightly further out:
1. Image/file distribution over low BW (10Mb) or lossy (wireless) networks.
......@@ -8,7 +8,7 @@ Things to do for image*:
an entire image checksum would be complicated by frisbee's out of order
receipt of chunks.
2. Imagezip could be multithread so that we can be reading ahead on the
2. Imagezip could be multithreaded so that we can be reading ahead on the
input device and overlapping IO with compression. Maybe a third thread
for doing output. Input is a little tricky since imagezip shortens up
its reads as it gets near the end of a chunk, so the buffer mechanism
......@@ -127,3 +127,54 @@ Things to do for image*:
efficiently recognize such blocks and whether we ultimately save space
over just allowing zlib to compress the data (presumably blocks of zeros
compress really well!)
11. Death to bubble sort and singly-linked lists.
We represent the set of free and allocated blocks with a singly-linked
list. The process is:
* Start with an empty "skips" (free) list.
* As we traverse all FSes on a disk, we identify free block ranges
which are appended to the skips list.
* The list is then sorted and abutting ranges merged.
We apply an optimization at this step where we throw away free
ranges that are less than a certain size. The logic is that, for
a couple of reasons, it is better to include a little bit of free
data in the image if it allows for a single larger contiguous
allocated range.
* The list is then inverted to get the "allocated" block list.
This we traverse in order to read the disk, compress, and make
the image.
So the most important operation is insert, and it isn't necessary to
maintain a sorted listed. However, it might prove to be practical to
keep it sorted if merge operations can be efficiently performed. An
alternative approach, to avoid the invert operation, would be to start
with a single allocated range (blocks 0 - sizeof_disk) and every time
we "add a free range" we are actually splitting the allocated range.
In other words, we keep a sorted, allocated block list from the beginning.
How big of lists do we need to worry about? It is hard to say,
realistically, how big a disk is reasonable to image with imagezip.
Attempting to image my 1TB disk at home yielded O(100,000) free block
ranges. Consider our current biggest disk at Emulab which is a 16TB array;
i.e., over 30 million sectors. Worst case is that every other sector
is free, so the list--be it "free" or "allocated"--would be around 15
million entries. So, whatever data structure we choose, should probably
handle O(1,000,000) and probably even 1-2 orders of magnitude larger.
How efficient does it need to be? Certainly not N**2 in the number of
list entries as it is now! Consider the 16TB array case above, and note
that saving such an image, with 8TB of valid data--even assuming say 10-1
compression and 250MB/sec sustained IO throughput--would take around an
hour. Having it take 1-5 minutes for list management would be acceptable.
Note 1: there is also a "reloc" list, which identifies ranges of
allocated blocks for which relocations must be performed in order to
keep imagezip data "position independent" so that, e.g., a Linux partition
image can be layed down anywhere on a disk. However, relocations are
infrequent enough that we don't necessarily have to optimize this list.
Note 2: back in ye ole OSKIT days, we created an "address map manager"
library which could apply here:
but that was still implemented with singly-linked lists.
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