Mysql – Disk write in iotop when doing thesql “Copy to tmp table”

In my.cnf use this option

tmpdir=/tmp (this is the default anyway, just choose another folder)

Restart of mysql required

BTW since the location.ID is '193' why even involve the location table ???

Try the following :

SELECT DISTINCT
    activity_seismo_info.ID, 
    activity_seismo_info.CREATED_AT, 
    activity_seismo_info.UPDATED_AT, 
    #....    
    activity_seismo_info.ASI_EXTRA_5, 
    seismo.ID, 
    #....    
    seismo.SEISMO_NOTES

FROM `activity_seismo_info`
    JOIN activity_info ON (activity_seismo_info.ASI_ACTIVITY_ID=activity_info.ID)
    JOIN (SELECT ID from location WHERE ID='193') location ON (activity_info.ACTIVITY_LOCATION_ID=location.ID)
    LEFT JOIN seismo ON (activity_seismo_info.ASI_SEISMO_ID=seismo.ID) 
;

For starters, I would not touch the buffer sizes just yet. The sizes youhave in the question are monstrously too big.

Here is another observation: You have BLOB data. Ouch, your temp table is going to eat space rather quickly. You could do somehting like this:

Create a 32GB RAM Disk called /var/tmpfs by adding this line to /etc/fstab

none                    /var/tmpfs              tmpfs   defaults,size=32g        1 2

Next, create a folder called /mysqltmp and mount the RAM disk on it

mkdir /mysqltmp
chown mysql:mysql /mysqltmp
mount /mysqltmp /var/tmpfs

Add this to my.cnf and restart mysql

[mysqld]
tmpdir=/mysqltmp

Now, any tmp table made via DDL lands in the RAM disk.

Here is yet another observation: Why not create a separate table that keeps the BLOB data away from the unique names?

CREATE TABLE `data_store_name` SELECT id,uniqname FROM `data_store` WHERE 1=2;
ALTER TABLE `data_store_name` ADD PRIMARY KEY (id);
ALTER TABLE `data_store_name` ADD UNIQUE KEY (uniqname);
ALTER TABLE `data_store_name` ADD INDEX name_id_ndx (uniqname,id);
INSERT INTO `data_store_name` SELECT id,uniqname FROM `data_store`;

This will prevent any moving around of BLOB data when indexing.

From here, you would have to always join data_store using its name like this:

SELECT
    A.uniqname,B.data
FROM
    (SELECT * FROM data_store_name WHERE uniqname = 'mydataname') A
    LEFT JOIN
    data_store B USING (id)
;

Making these changes will sidestep this whole mess of dealing with keycache, RAM disks, and tmp tables.

Give it a Try !!!