Mr. Padilha,
In general, what you are trying to do is discouraged. It is possible to open and read/write files from kernel space, but the architecture is not very portable. I actually do this all the time, but only for debugging traces.
I would recommend another implementation method if you want to implement a block device backed by files. The "Network Block Device" is a project that includes a kernel module (nbd.ko) and a user-space server. The kernel module is usable without the user space module and lets you implement a decent, multi-threaded, block device completely from user space code.
nbd.ko is reached by opening a master device (which is a character device) and then reading/writing commands into the single file handle. Operations are "async" so they multi-thread easily and you can easily implement all of the block device functions including new stuff like discard.
Normally, the user-space nature of this would be a performance hit, but if you are using the "cloud", the extra latency might not matter much.
Doug Dumitru
EasyCo LLC
On Sun, Aug 16, 2015 at 9:55 PM, Thiago Padilha <tpadilha84@xxxxxxxxx> wrote:
- block 2 8192- block 1: 4096- block 0: 0After this command is executed, there's a new file with 4k size at /dirdm/0. This file will be used to store the first block of the device. Here are the filenames that represent some blocks of this virtual device:dirdm create --size 10G --block-size 4k /dirdmHiFirst of all, I'm very new on the subject(kernel programming/device mappers/block devices), so forgive if I say anything stupid.I need a device mapper target that stores data in files of fixed sizes, probably defined when the virtual device is first created. For the sake of explanation, lets call this target as "dirdm" and assume there's an userspace tool of the same name that can be used to manage such virtual devices. Now let's say I have an empty directory "/dirdm" and want to create a virtual device with 4k "block size" on top of it:- block 3: 12288- block 4: 16384That is, each block is stored in a file named after the first byte on the block. Querying a block that has no data written will simply return 0(block files are also initialized with 0), so it behaves like a sparse block device.Is there any device mapper implementation that manages data in a similar manner? Note that these details about filenames or directory structure are not important, all I care about is that the dm target splits data in relatively small files on a directory.The reason for these requirements is to be able to efficiently store arbitrary block devices in a cloud storage service such a google drive/onedrive without syncing a big loop device on every modification(Only files representing affected blocks are modified). One useful application would be to create a big device(such as 100g size) and add luks/ext4 on top of it, which would be a great way to store private data.If there are no existing dm targets matching these requirements, what would be a recommended reading/documentation to get started on implementing this on my own?
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Doug Dumitru
EasyCo LLC
EasyCo LLC
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