When using qemu-img to convert an image that is hosted on an HTTP server to some faster local (or pseudo-local) storage, the overall performance can be improved by reading data from the HTTP server in larger blocks and by caching and re-using blocks already read. This set of patches implements both of these, and adds a further patch allowing an offset to be added to all of the HTTP requests.
The first patch (block/curl: Add an 'offset' parameter, affecting all range requests) allows the user to add an arbitrary offset to all range requests sent to the HTTP server. This is useful if the image to be read from the HTTP server is embedded in another file (for example an uncompressed tar file). It avoids the need to first download the file containing the source image and extract it (both of which will require writing the image to local storage). It is logically distinct from the rest of the patches and somewhat use-case specific. The remaining patches implement block based retrieval of data from the HTTP server and, optionally, caching of those blocks in memory. The existing HTTP implementation simply reads whatever data is requested by the caller, with the option for a user-specified amount of readahead. This is poor for performance because most IO requests (from QCOW2, for example) are for relatively small amounts of data, typically no more than 64kB. This does not allow the underlying TCP connections to achieve peak throughput. The existing readhead mechanism is also intended to work in conjunction with the HTTP driver's attempt to piggy-back a new IO request on one that is already in flight. This works, but is often defeated because it relies on the existing IO request *completely* satisfying any subsequent request that might piggy-back onto it. This is rarely the case and, particularly when used with "readahead", can result in the same data being downloaded repeatedly. The observed performance will depend greatly on the environment, but when using qemu-img to retrieve a 1GiB QCOW2 image from an HTTPS server, the following was observed: | approach | time (hh:mm:ss) | |--------------------------------------------+-----------------| | QCOW2 over HTTPS (existing implementation) | 00:00:59 | | 256kB blocks, 8 cached blocks | 00:00:42 | | 2MB blocks, 100 cached blocks | 00:00:34 | By way of comparison, aria2c (a dedicated HTTP download client) can retrieve the same image in 19 seconds. Obviously this is without any QCOW2 layer. David Edmondson (9): block/curl: Add an 'offset' parameter, affecting all range requests block/curl: Remove readahead support block/curl: Tracing block/curl: Perform IO in fixed size chunks block/curl: Allow the blocksize to be specified by the user block/curl: Cache downloaded blocks block/curl: Allow the user to control the number of cache blocks block/curl: Allow 16 sockets/ACB block/curl: Add readahead support block/curl.c | 515 ++++++++++++++++++++++---- block/io.c | 4 + block/linux-aio.c | 6 + block/trace-events | 18 +- docs/system/device-url-syntax.rst.inc | 15 + qapi/block-core.json | 11 +- 6 files changed, 488 insertions(+), 81 deletions(-) -- 2.27.0
