SeaWeedfs学习总结

SeaWeedfs最初是作为一个对象存储来有效地处理小文件的。central master不管理central master中的所有文件元数据，而只管理文件卷，它通过这些volume servers管理文件及其元数据。这减轻了来自central master的并发压力，并将文件元数据分散到卷服务器中，从而允许更快的文件访问（O（1），通常只有一个磁盘读取操作）。每个文件的元数据只有40字节的磁盘存储开销。

1 架构

官网描述：
Usually distributed file systems split each file into chunks, a central master keeps a mapping of filenames, chunk indices to chunk handles, and also which chunks each chunk server has.
The main drawback is that the central master can’t handle many small files efficiently, and since all read requests need to go through the chunk master, so it might not scale well for many concurrent users.
Instead of managing chunks, SeaweedFS manages data volumes in the master server. Each data volume is 32GB in size, and can hold a lot of files. And each storage node can have many data volumes. So the master node only needs to store the metadata about the volumes, which is a fairly small amount of data and is generally stable…

seaWeedFs的文件存储的基本结构如上，我们从下往上看
数据卷：也就是是类似我们windowspan的D,E盘等物理磁盘，文件保存的物理介质。默认32GB，通过修改代码可以修改为64GB或者128GB,每个文件的最大不超过单个卷的大小。
官网描述：In the current implementation, each volume can hold 32 gibibytes (32GiB or 8x2^32 bytes). This is because we align content to 8 bytes. We can easily increase this to 64GiB, or 128GiB, or more, by changing 2 lines of code, at the cost of some wasted padding space due to alignment.
There can be 4 gibibytes (4GiB or 2^32 bytes) of volumes. So the total system size is 8 x 4GiB x 4GiB which is 128 exbibytes (128EiB or 2^67 bytes).
Each individual file size is limited to the volume size.
数据卷服务器：为了更方便的管理数据卷，通过该服务来对多个数据卷进行管理。其中，数据卷服务器保存了文件元数据，通过访问文件元数据就可以操作数据卷中的文件。从官网中我们了解到文件的元数据只有16字节大小。【文件句柄：表示文件对象的一个便于使用的引用】
官网描述：The actual data is stored in volumes on storage nodes. One volume server can have multiple volumes, and can both support read and write access with basic authentication. 一个卷服务对应多个卷
The actual file metadata is stored in each volume on volume servers. Since each volume server only manages metadata of files on its own disk, with only 16 bytes for each file, all file access can read file metadata just from memory and only needs one disk operation to actually read file data.每个文件的元数据16字节大小，Linu