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Q: RAID1 and chunk size

I have a RAID1 array with 2 disks (/dev/sda1 and /dev/sdb1) of 2 TB
each.  By running mdadm -X /dev/sda1 I see that the chunk size is 64 MB:

    # mdadm -X /dev/sda1
    Filename : /dev/sda1
    Magic : 6d746962
    Version : 4
    UUID : 300551ed:f6690dfb:1c939898:af5509c6
    Events : 2599997
    Events Cleared : 2599997
    State : OK
    Chunksize : 64 MB
    Daemon : 5s flush period
    Write Mode : Normal
    Sync Size : 1953381376 (1862.89 GiB 2000.26 GB)
    Bitmap : 29807 bits (chunks), 2 dirty (0.0%)

What exactly does the chunk sized mean?  My question is how reads and
writes on an array are done.  Will the kernel always read or write a
complete chunk?  If so, does that mean that writing a single 4 KB
block to a file system will cause a 64 MB read, i.e one chunk, change
the 4 KB block in that chunk and write back the 64 MB chunk?

Wouldn't that mean a massive performance problem?